Fix #28601: SEGFAULT: Regression in free_imbuf_seq

Actually, it's NULL-pointer dereference in rna_Sequence_update caused by RNA cache.

Discussed with Joshua and he thought it's acceptable for now to add
extra NULL-check here.

source/blender/makesrna/intern/rna_sequencer.c

@@ -559,6 +559,7 @@ static void rna_Sequence_update(Main *UNUSED(bmain), Scene *scene, PointerRNA *p
 {
 	Editing *ed= seq_give_editing(scene, FALSE);
 
+	if(ed)
 		free_imbuf_seq(scene, &ed->seqbase, FALSE, TRUE);
 }