bug#24614: 25.1.50; Exception after moving mouse over over a different window

[Eli Zaretskii]

> From: Alain Schneble <a.s@realize.ch>
> CC: <24614@debbugs.gnu.org>
> Date: Tue, 4 Oct 2016 20:06:57 +0200
> 
> (gdb) p w->output_cursor
> $7 = {
>   x = 0, 
>   y = 280, 
>   hpos = -1, <<<<<<<<<<<<<<<<<
>   vpos = 20
> }

Thanks, the problem is now acutely clear: we were indexing the glyphs
array using a negative index.  I wonder why it took so many years for
this bug to manifest itself.

I pushed a fix, and am marking this bug done.  Feel free to reopen if
there are left-overs.

Thanks.