Caused by rBaf162658e127: Texture painting:, so long standing bug.

When changing clone slots (report involved a quite complicated sequence
of selecting textures and undo -- but I think this could happen in more
situations) code checks for UV of new clone slot.
However, since above commit the slot and the clone slot were mixed up,
so in this case the responsible NULL check (for when no UV is assigned)
wasnt working.
Now correct this (NULL check the clone slot UV -- instead of the paint
slot UV).

note: not sure why low level CustomData functions actually dont do the
name NULL checks themselves (seems like callers are always responsible).