Blender Git Loki

BKE_callback_remove: prevent crash on Blender exit

`BKE_callback_remove` now checks whether the callback actually is known,
before trying to remove it.

`BKE_blender_atexit()` runs after `BKE_callback_global_finalize()`. When
an at-exit callback tried to unregister its BKE callbacks, these would
already be unregistered, causing a crash of Blender when exiting,