util: define initializer for 'struct ucred' that properly invalidates all fields

i.e. let's make sure to invalid uid/gid to UID_INVAID + GID_INVALID
instead of zero.

src/shared/varlink.c

@@ -258,8 +258,7 @@ static int varlink_new(Varlink **ret) {
 
                 .state = _VARLINK_STATE_INVALID,
 
-                .ucred.uid = UID_INVALID,
-                .ucred.gid = GID_INVALID,
+                .ucred = UCRED_INVALID,
 
                 .timestamp = USEC_INFINITY,
                 .timeout = VARLINK_DEFAULT_TIMEOUT_USEC
@@ -2106,8 +2105,8 @@ static int count_connection(VarlinkServer *server, struct ucred *ucred) {
 
 int varlink_server_add_connection(VarlinkServer *server, int fd, Varlink **ret) {
         _cleanup_(varlink_unrefp) Varlink *v = NULL;
+        struct ucred ucred = UCRED_INVALID;
         bool ucred_acquired;
-        struct ucred ucred;
         int r;
 
         assert_return(server, -EINVAL);