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,

source/blender/blenkernel/intern/callbacks.c

@@ -82,7 +82,12 @@ void BKE_callback_add(bCallbackFuncStore *funcstore, eCbEvent evt)
 void BKE_callback_remove(bCallbackFuncStore *funcstore, eCbEvent evt)
 {
   ListBase *lb = &callback_slots[evt];
-  BLI_remlink(lb, funcstore);
+
+  /* Be safe, as the callback may have already been removed by BKE_callback_global_finalize(), for
+   * example when removing callbacks in response to a BKE_blender_atexit_register callback
+   * function. `BKE_blender_atexit()` runs after `BKE_callback_global_finalize()`. */
+  BLI_remlink_safe(lb, funcstore);
+
   if (funcstore->alloc) {
     MEM_freeN(funcstore);
   }