bug#50406: fill-paragraph error

[Juri Linkov]

>> Please try to open the attached file in emacs -Q, and after moving point
>> to the beginning of the 3rd line, type 'M-q'.  When debug-on-error is t,
>> it fails with such backtrace:
>>
>> Debugger entered--Lisp error: (error "Invalid search bound (wrong side
>> of point)")
>>   re-search-backward("[ \11]\\|\\c|.\\|.\\c|" 280 0)
>>   fill-move-to-break-point(280)
>>   fill-region-as-paragraph(184 210 nil nil 186)
>>   fill-comment-paragraph(nil)
>>   fill-paragraph(nil t)
>>   funcall-interactively(fill-paragraph nil t)
>>   command-execute(fill-paragraph)
>
> This doesn't happen now in Emacs 29.  Instead I get:
>
> abc abc abc abc abc abc abc abc abc abc abc abc abc abc abc abc abc abc abc 
> abc abc abc abc ; def
>                                                                               
>             ; def
>                                                                               
>             ; def
>                                                                               
>             ; def
>                                                                               
>             ; def
>                                                                               
>             ; ghi
>
> Is that the result we're supposed to get, though?  I'm not at all sure
> how fill-paragraph/comment-start are supposed to interact here.

This is what it always did in all earlier versions until the recent
breakage that is fixed now.