tests/qtest: Free unused QMP response

This fixes LeakSanitizer warnings.

Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Message-ID: <20240627-san-v2-11-750bb0946dbd@daynix.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

tests/qtest/libqtest.c

@@ -749,6 +749,8 @@ QDict *qtest_qmp_receive(QTestState *s)
                         response, s->eventData)) {
             /* Stash the event for a later consumption */
             s->pending_events = g_list_append(s->pending_events, response);
+        } else {
+            qobject_unref(response);
         }
     }
 }