Bug in LaTeX-fill-paragraph

Start emacs -q, add auctex to the load path, and open an empty buffer
in LaTeX-mode.  Type ~~foo.  Now hit M-q, and the text is unchanged,
as it should be.  Now put one or two spaces in front, so that it is:

  ~~foo

Now do M-q again.  The two spaces disappear, as they should, but the
two non-breaking spaces disappear as well!

AUCTeX-version's value is "11.54"

This bug bit me badly, when it removed some manual tweaking I had done
to the formatting of a document, and of course I didn't notice it at
the time.  I don't think that filling should ever remove ~ in any
circumstances.

Best wishes,

Peter