fill-paragraph bug

[Chong Yidong]

There is an error condition somewhere in fill-paragraph that makes
subsequent calls to that command unusable.

Insert the following text in a LaTeX buffer, and call M-q:

AAAAAAAAAAAAAA AAAAAAAAAAAAAAAAA AAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA 
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA 
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA 
AAAAAAAAAAAAAAAAA%AAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA 
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAA

the result:

Debugger entered--Lisp error: (error "fill-prefix too long for specified width")
  signal(error ("fill-prefix too long for specified width"))
  error("fill-prefix too long for specified width")
  fill-delete-prefix(...)
  fill-region-as-paragraph(1 323 nil nil 227)
  fill-comment-paragraph(nil)
  fill-paragraph(nil)
  call-interactively(fill-paragraph)

Now try to fill any other paragraph anywhere else in Emacs.  This
triggers an error:

"fill-prefix too long for specified width"

Probably some variable somewhere needs to be put in a condition-case.