CHAOS: cleanup possible null vars on msg alloc failure

In message.c, if msg_alloc fails to init the string field,
vars may be null, so use a null tolerant cleanup.

In res_pjsip_messaging.c, if msg_data_create fails, mdata
will be null, so use a null tolerant cleanup.

ASTERISK-25323

Change-Id: Ic2d55c2c3750d5616e2a05ea92a19c717507ff56

main/message.c

@@ -398,7 +398,7 @@ static void msg_destructor(void *obj)
 	struct ast_msg *msg = obj;
 
 	ast_string_field_free_memory(msg);
-	ao2_ref(msg->vars, -1);
+	ao2_cleanup(msg->vars);
 }
 
 struct ast_msg *ast_msg_alloc(void)

res/res_pjsip_messaging.c

@@ -597,7 +597,7 @@ static int sip_msg_send(const struct ast_msg *msg, const char *to, const char *f
 
 	if (!(mdata = msg_data_create(msg, to, from)) ||
 	    ast_sip_push_task(message_serializer, msg_send, mdata)) {
-		ao2_ref(mdata, -1);
+		ao2_cleanup(mdata);
 		return -1;
 	}
 	return 0;