One small addition to 294384 found while very carefully merging to 1.6.

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@294641 65c4cc65-6c06-0410-ace0-fbb531ad65f3

main/pbx.c

@@ -4058,6 +4058,12 @@ void ast_merge_contexts_and_delete(struct ast_context **extcontexts, const char
 				continue;
 			}
 			ao2_lock(hint);
+
+			if (hint->exten == NULL) {
+				ao2_unlock(hint);
+				continue;
+			}
+
 			this->callbacks = hint->callbacks;
 			hint->callbacks = NULL;
 			this->laststate = hint->laststate;