ikev2: Don't increase expected MID after handling MID sync message

If the responder never sent a message the expected MID is 0.  While
the sent MID (M1) SHOULD be increased beyond the known value, it's
not necessarily the case.
Since M2 - 1 would then equal UINT_MAX setting that MID would get ignored
and while we'd return 0 in the notify we'd actually expect 1 afterwards.

src/libcharon/sa/ikev2/task_manager_v2.c

@@ -918,6 +918,9 @@ static status_t build_response(private_task_manager_t *this, message_t *request)
 		/* we don't want to resend messages to sync MIDs if requests with the
 		 * previous MID arrive */
 		clear_packets(this->responding.packets);
+		/* avoid increasing the expected message ID after handling a message
+		 * to sync MIDs with MID 0 */
+		return NEED_MORE;
 	}
 
 	array_compress(this->passive_tasks);