gparam: fix memory leak in g_param_value_defaults()
We cannot just call
G_PARAM_SPEC_GET_CLASS (pspec)->value_set_default (pspec, &dflt_value);
without initializing the GValue first. It would call param_string_set_default(), which would set the pointer value to a cloned string (which later never gets released, because the GValue is not known to hold a string).