[Freeze break request] gtksourceview crash

[Paolo Borelli <paolo borelli gmail com>]

Hi,

	I'd like to apply the small patchset attached to

https://bugzilla.gnome.org/show_bug.cgi?id=595450

The fix is straightforward, it affects a new api added in this
development cycle that was not tested under all conditions, preventing a
crash and ugly empty tooltips when no tooltip for the GtkSourceMark is
set.


Ciao
	Paolo