tests: test-qmp-commands: Fix double free

The ret variable is freed twice, but on the second time we actually want
to free ret3 instead. Don't know why this didn't explode.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Tested-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>

tests/test-qmp-commands.c

@@ -141,7 +141,7 @@ static void test_dispatch_cmd_io(void)
 
     ret3 = qobject_to_qint(test_qmp_dispatch(req));
     assert(qint_get_int(ret3) == 66);
-    QDECREF(ret);
+    QDECREF(ret3);
 
     QDECREF(req);
 }