bug#50096: args-out-of-range in redisplay_internal

[Eli Zaretskii]

> From: Juri Linkov <juri@linkov.net>
> Cc: 50096@debbugs.gnu.org
> Date: Tue, 17 Aug 2021 21:24:29 +0300
> 
> (gdb) p BUF_BEGV (XBUFFER (object))
> $2 = 1
> (gdb) p BUF_ZV (XBUFFER (object))
> $3 = 1

So we are in the wrong (empty) buffer?  Or did someone delete the
buffer's text under our feet?  What does this show:

  (gdb) p object->name_
  (gdb) xstring
  (gdb) p current_buffer->name_
  (gdb) xstring