[0.9.9] Run-time err on next NG

Hi. I might have found a small bug in slrn 0.9.9. Here's how to
reproduce it:

1. Read all the downloaded articles in a newsgroup, when asked to press
   'n' or 'space' for next group, do so;

2. slrn asks how many articles to download for the next group - reply
   with a letter;

3. slrn goes back to newsgroup list and says "Run-Time Error".

Probably just some trivial input validation issue.


-----BEGIN VERSION INFO-----
~> slrn --version
slrn 0.9.9
S-Lang Library Version: 2.1.3
Compiled at: Aug 18 2008 13:32:51
Operating System: Linux

COMPILE TIME OPTIONS:
Backends: +nntp +slrnpull +spool
External programs / libs: -canlock -inews -ssl -uudeview +iconv
Features: +decoding +emphasized_text +end_of_thread +fake_refs
+gen_msgid
-grouplens -msgid_cache +piping +rnlock +spoilers -strict_from
Using 64 bit integers for article numbers.

DEFAULTS:
Default server object:     nntp
Default posting mechanism: nntp


-- 
~> cat /etc/redhat-release && uname -a
Scientific Linux SL release 5.2 (Boron)
Linux acerrimo.local 2.6.18-92.1.22.el5 #1 SMP Tue Dec 16 06:50:02 EST 
2008 i686 athlon i386 GNU/Linux


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