Fix a sporadic segfault in treeview keynav
Fix a sporadic segfault in treeview keynav

If a a treeview has frequent periodic additions and removals of
rows, it is possible that a page down keypress moves the cursor
out of the height of the treeview. In some of these cases, we
can be tricked into dereferencing a  NULL pointer.

Bug 612919.

https://bugzilla.gnome.org/show_bug.cgi?id=746269

Distrotech/gtk,Distrotech/gtk,Distrotech/gtk,Distrotech/gtk,Distrotech/gtk