src/clients/lib/xmmsclient/visualization/unixshm.c: fix -std=c23 build

gcc-15 switched to -std=c23 by default:

    https://gcc.gnu.org/git/?p=gcc.git;a=commitdiff;h=55e3bd376b2214e200fa76d12b67ff259b06c212

Without the change the build fails as:

    ../src/clients/lib/xmmsclient/visualization/unixshm.c:31:24: error: incompatible types when returning type ‘_Bool’ but ‘xmmsc_result_t *’ {aka ‘struct xmmsc_result_St *’} was expected
       31 |                 return false;
          |                        ^~~~~

src/clients/lib/xmmsclient/visualization/unixshm.c

@@ -28,7 +28,7 @@ setup_shm_prepare (xmmsc_connection_t *c, int32_t vv)
 	t->shmid = shmget (IPC_PRIVATE, sizeof (xmmsc_vischunk_t) * XMMS_VISPACKET_SHMCOUNT, S_IRWXU + S_IRWXG + S_IRWXO);
 	if (t->shmid == -1) {
 		c->error = strdup ("Couldn't create the shared memory!");
-		return false;
+		return NULL;
 	}
 	/* attach early, so that the server doesn't think we aren't there */
 	buffer = shmat(t->shmid, NULL, SHM_RDONLY);