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

[martin rudalics]

> After setting a breakpoint at 5192, the segfault would trigger without
> the breakpoint kicking in. I then moved that hunk up to line 5172 (diff
> below). Now the breakpoint there would kick in, and further, now
                                                               ^^^
Hopefully, that's a "no" up there.

> segfault would trigger; instead, it gave the «error while displaying
> tooltip message», and fell back on showing the tooltip message in the
> echo area. Below is the state of  «image_cache_refcount» for a few
> breaks. I fumbled a bit with the line history and only printed it for
> some of the breaks.

martin