dbus_test_tool_spam: Fix 'variable payload going out of scope leaks the storage it points to' (CID 54759)

Reported by Coverity: CID 54759: Resource leak (RESOURCE_LEAK)

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=90021
Reviewed-by: Simon McVittie <simon.mcvittie@collabora.co.uk>

tools/dbus-spam.c

@@ -368,6 +368,7 @@ dbus_test_tool_spam (int argc, char **argv)
                        error.name, error.message);
               dbus_error_free (&error);
               dbus_free (random_sizes);
+              dbus_free (payload_buf);
               return 1;
             }