bug#47338: 28.0.50; Unexpected error in fill-region

[Eli Zaretskii <eliz@gnu.org>]

> From: Evgeny Zajcev <lg.zevlg@gmail.com>
> Date: Tue, 23 Mar 2021 12:43:20 +0300
> 
> Evaluating next sexp results in error:
>   (with-temp-buffer
>     (let ((fill-column 66))
>       (insert "                                   :action (lambda (docid msg target)")
>       (fill-region (point-min) (point-max) 'left t)
>       (buffer-string)))
>   
> Here is the backtrace:
>   Debugger entered--Lisp error: (error "Invalid search bound (wrong side of point)")
>     re-search-backward("[ \11]\\|\\c|.\\|.\\c|" 71 0)
>     fill-move-to-break-point(71)
>     fill-region-as-paragraph(1 70 left t)
>     fill-region(1 70 left t)
>     (let ((fill-column 66)) (insert "                                   :action (lambda...") (fill-region (point-min) (point-max)
> 'left t) (buffer-string))
>   ...

This is the result of fixing bug#45720.  In the above scenario, the
first time through the loop LINEBEG in the fragment below is set to
EOB+1 instead of the point where the fill-prefix ends:

	(let ((first t)
              linebeg)
	  (while (< (point) to)
            ;; On the first line, there may be text in the fill prefix
            ;; zone.  In that case, don't consider that area when
            ;; trying to find a place to put a line break (bug#45720).
            (if (not first)
	        (setq linebeg (point))
              (setq first nil
                    linebeg (+ (point) (length fill-prefix)))) <<<<<<<<<<

CC'ing Lars who made that change.