Bug 478519 - GtkTooltip segfaults on NULL gdk-display-current-tooltip

Bug 478519 -  GtkTooltip segfaults on NULL gdk-display-current-tooltip

Make the tooltip code a bit more robust for a case that only occurs when
GTK+ is used from a language binding.  It looks like this case appears
because the memory management / ref counting is handled differently in
some of the language bindings.  Instead of asserting, we will fail
silently.  Also fix a think-o in gtk_tooltip_start_delay().  Patch from
O. Andrieu.