st: Fix crash in theme-node-transition

Setting up the framebuffers for transitions may fail, in which case
the material used for drawing is left uninitialized, so trying to
access it results in a crash.
Instead bail out in this case, which means that we won't paint
anything during the transition - still, drawing errors are better
than crashes ...

https://bugzilla.gnome.org/show_bug.cgi?id=659676