bug#40661: Crash in regex search during redisplay

[Eli Zaretskii]

> Date: Thu, 16 Apr 2020 18:36:36 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: 40661@debbugs.gnu.org
> 
> Looks like GC sometimes kicks in while we are inside re_search_2

Or not.  I cannot get a breakpoint inside GC to fire while we are in
search_buffer_re, so maybe my hypothesis was wrong.  Although the
symptoms are all there: when the segfault hits, the pointers passed to
re_search_2 are invalid, but BEGV_ADDR and GAP_END_ADDR, from which
they are supposed to be computed, are valid (and different).  And the
patch does seem to avoid the segfaults.  But maybe it's just a
coincidence or a side effect...