[emacs-w3m:13510] Bug in w3m--get-page-anchors

Hi,

(I'm directly CC'ing Yamaoka because the last time I tried to post to
this group something went wrong).

This is Emacs master (doesn't matter AFAICT) with recent w3m installed
from Melpa, 20190801.156 (does matter, the problem is quite new).

Recipe: I open some page, say, https://de.wikipedia.org/wiki/Lie-Gruppe (also
doesn't matter), click on any anchor and get this error:

Debugger entered--Lisp error: (wrong-type-argument stringp nil)
  string-match("\\`cite_.*[0-9]\\'" nil)
  #f(compiled-function (x) #<bytecode 0x1559a52f6745>)((nil . 29925))
  cl-remove(nil (("p-search" . 29926) (nil . 29925) ("p-search" . 29918) (nil . 29916) ("p-search" . 29910) (nil . 29909) ("p-search" . 29880) ("mw-head" . 29613) ("Weblinks" . 28992) ("Literatur" . 26591) ("cite_note-8" . 26571) ("cite_note-hawkins-p100-7" . 26543) ("cite_note-hawkins-p43-6" . 26516) ("cite_note-hawkins-p76-5" . 26489) ("cite_note-hawkins-p2-4" . 26463) ("cite_note-hawkins-p1-3" . 26431) ("cite_note-2" . 25259) ("cite_note-1" . 25164) ("Anmerkungen" . 25116) ("Klassifikationsmöglichkeiten" . 23571) ("Differentialgeometrie_von_Lie-Gruppen" . 21971) ("cite_ref-8" . 21823) ("cite_ref-hawkins-p100_7-0" . 21010) ("cite_ref-hawkins-p43_6-0" . 19543) ("cite_ref-hawkins-p76_5-0" . 18227) ("cite_ref-hawkins-p2_4-0" . 17957) ("cite_ref-hawkins-p1_3-1" . 17696) ("cite_ref-hawkins-p1_3-0" . 17401) ("Frühgeschichte" . 17281) ("Beispiele" . 13894) ("Lie-Untergruppe" . 13211) ("Lie-Gruppen-Homomorphismus" . 12055) ("Lie-Algebra_der_Lie-Gruppe" . 9284) ("Lie-Gruppe" . 8637) ("Definitionen" . 8588) ("Erste_Beispiele" . 1775) ("cite_ref-2" . 738) ("cite_ref-1" . 263) ("top" . 105) ("footer-poweredbyico" . 31618) ("footer-copyrightico" . 31606) ("footer-icons" . 31602) ("footer-places-mobileview" . 31586) ("footer-places-cookiestatement" . 31557) ("footer-places-developers" . 31542) ("footer-places-disclaimer" . 31528) ("footer-places-about" . 31509) ("footer-places-privacy" . 31493) ("footer-places" . 31489) ("footer-info-copyright-info" . 30951) ...) :if #f(compiled-function (x) #<bytecode 0x1559a52f6745>))
  apply(cl-remove nil (("p-search" . 29926) (nil . 29925) ("p-search" . 29918) (nil . 29916) ("p-search" . 29910) (nil . 29909) ("p-search" . 29880) ("mw-head" . 29613) ("Weblinks" . 28992) ("Literatur" . 26591) ("cite_note-8" . 26571) ("cite_note-hawkins-p100-7" . 26543) ("cite_note-hawkins-p43-6" . 26516) ("cite_note-hawkins-p76-5" . 26489) ("cite_note-hawkins-p2-4" . 26463) ("cite_note-hawkins-p1-3" . 26431) ("cite_note-2" . 25259) ("cite_note-1" . 25164) ("Anmerkungen" . 25116) ("Klassifikationsmöglichkeiten" . 23571) ("Differentialgeometrie_von_Lie-Gruppen" . 21971) ("cite_ref-8" . 21823) ("cite_ref-hawkins-p100_7-0" . 21010) ("cite_ref-hawkins-p43_6-0" . 19543) ("cite_ref-hawkins-p76_5-0" . 18227) ("cite_ref-hawkins-p2_4-0" . 17957) ("cite_ref-hawkins-p1_3-1" . 17696) ("cite_ref-hawkins-p1_3-0" . 17401) ("Frühgeschichte" . 17281) ("Beispiele" . 13894) ("Lie-Untergruppe" . 13211) ("Lie-Gruppen-Homomorphismus" . 12055) ("Lie-Algebra_der_Lie-Gruppe" . 9284) ("Lie-Gruppe" . 8637) ("Definitionen" . 8588) ("Erste_Beispiele" . 1775) ("cite_ref-2" . 738) ("cite_ref-1" . 263) ("top" . 105) ("footer-poweredbyico" . 31618) ("footer-copyrightico" . 31606) ("footer-icons" . 31602) ("footer-places-mobileview" . 31586) ("footer-places-cookiestatement" . 31557) ("footer-places-developers" . 31542) ("footer-places-disclaimer" . 31528) ("footer-places-about" . 31509) ("footer-places-privacy" . 31493) ("footer-places" . 31489) ("footer-info-copyright-info" . 30951) ...) :if #f(compiled-function (x) #<bytecode 0x1559a52f6745>) nil)
  cl-remove-if(#f(compiled-function (x) #<bytecode 0x1559a52f6745>) (("p-search" . 29926) (nil . 29925) ("p-search" . 29918) (nil . 29916) ("p-search" . 29910) (nil . 29909) ("p-search" . 29880) ("mw-head" . 29613) ("Weblinks" . 28992) ("Literatur" . 26591) ("cite_note-8" . 26571) ("cite_note-hawkins-p100-7" . 26543) ("cite_note-hawkins-p43-6" . 26516) ("cite_note-hawkins-p76-5" . 26489) ("cite_note-hawkins-p2-4" . 26463) ("cite_note-hawkins-p1-3" . 26431) ("cite_note-2" . 25259) ("cite_note-1" . 25164) ("Anmerkungen" . 25116) ("Klassifikationsmöglichkeiten" . 23571) ("Differentialgeometrie_von_Lie-Gruppen" . 21971) ("cite_ref-8" . 21823) ("cite_ref-hawkins-p100_7-0" . 21010) ("cite_ref-hawkins-p43_6-0" . 19543) ("cite_ref-hawkins-p76_5-0" . 18227) ("cite_ref-hawkins-p2_4-0" . 17957) ("cite_ref-hawkins-p1_3-1" . 17696) ("cite_ref-hawkins-p1_3-0" . 17401) ("Frühgeschichte" . 17281) ("Beispiele" . 13894) ("Lie-Untergruppe" . 13211) ("Lie-Gruppen-Homomorphismus" . 12055) ("Lie-Algebra_der_Lie-Gruppe" . 9284) ("Lie-Gruppe" . 8637) ("Definitionen" . 8588) ("Erste_Beispiele" . 1775) ("cite_ref-2" . 738) ("cite_ref-1" . 263) ("top" . 105) ("footer-poweredbyico" . 31618) ("footer-copyrightico" . 31606) ("footer-icons" . 31602) ("footer-places-mobileview" . 31586) ("footer-places-cookiestatement" . 31557) ("footer-places-developers" . 31542) ("footer-places-disclaimer" . 31528) ("footer-places-about" . 31509) ("footer-places-privacy" . 31493) ("footer-places" . 31489) ("footer-info-copyright-info" . 30951) ...))
  w3m--filter-page-anchors((("p-search" . 29926) (nil . 29925) ("p-search" . 29918) (nil . 29916) ("p-search" . 29910) (nil . 29909) ("p-search" . 29880) ("mw-head" . 29613) ("Weblinks" . 28992) ("Literatur" . 26591) ("cite_note-8" . 26571) ("cite_note-hawkins-p100-7" . 26543) ("cite_note-hawkins-p43-6" . 26516) ("cite_note-hawkins-p76-5" . 26489) ("cite_note-hawkins-p2-4" . 26463) ("cite_note-hawkins-p1-3" . 26431) ("cite_note-2" . 25259) ("cite_note-1" . 25164) ("Anmerkungen" . 25116) ("Klassifikationsmöglichkeiten" . 23571) ("Differentialgeometrie_von_Lie-Gruppen" . 21971) ("cite_ref-8" . 21823) ("cite_ref-hawkins-p100_7-0" . 21010) ("cite_ref-hawkins-p43_6-0" . 19543) ("cite_ref-hawkins-p76_5-0" . 18227) ("cite_ref-hawkins-p2_4-0" . 17957) ("cite_ref-hawkins-p1_3-1" . 17696) ("cite_ref-hawkins-p1_3-0" . 17401) ("Frühgeschichte" . 17281) ("Beispiele" . 13894) ("Lie-Untergruppe" . 13211) ("Lie-Gruppen-Homomorphismus" . 12055) ("Lie-Algebra_der_Lie-Gruppe" . 9284) ("Lie-Gruppe" . 8637) ("Definitionen" . 8588) ("Erste_Beispiele" . 1775) ("cite_ref-2" . 738) ("cite_ref-1" . 263) ("top" . 105) ("footer-poweredbyico" . 31618) ("footer-copyrightico" . 31606) ("footer-icons" . 31602) ("footer-places-mobileview" . 31586) ("footer-places-cookiestatement" . 31557) ("footer-places-developers" . 31542) ("footer-places-disclaimer" . 31528) ("footer-places-about" . 31509) ("footer-places-privacy" . 31493) ("footer-places" . 31489) ("footer-info-copyright-info" . 30951) ...))
  w3m--get-page-anchors()
  [...]
  w3m-goto-url("https://de.wikipedia.org/wiki/Lie-Gruppe#Erste_Bei..." nil)
  w3m-view-this-url(nil nil)

Note the ill alist `w3m--filter-page-anchors' receives: every second key
is nil.  It is from `w3m--get-page-anchors': see this loop:

(while (setq pos (next-single-property-change pos 'w3m-name-anchor))
	(push (cons (car (get-text-property pos 'w3m-name-anchor)) pos)
	      anchor-list))

`next-single-property-change' also stops where the propertized text
ends, I think this is where the nil entries come from.

TIA, Michael.