bug#20802: Segfault when showing non-GTK+ tooltip

[martin rudalics]

> This was after setting x-gtk-use-system-tooltip to nil and customizing
> the tooltip face to #qqq.
[...]
> Program received signal SIGSEGV, Segmentation fault.
> cache_image (f=0x1170a68, img=<optimized out>) at image.c:1782
> 1782         for (i = 0; i < c->used; ++i)

This should be indeed the same segfault I saw.  Meanwhile I checked in a
fix on trunk/master.  If you can build Emacs please try it.

Thanks for report and backtrace, martin