Blender Git Loki

Fix T66030: [CRASH] Modifying Normals with Skin Modifier.

clnor editing code was simply not checking at all whether it has
something to work on... Guess nobody had idea to edit custom normals on
a mesh that has no normals before! :P

This should probably be handled in a poll function too, to completely
disable those tools when there are no faces/loops, but let's keep it to
minimal changes at that point.