remove unneeded check of `m` in arrange()

arrange must never be called with a NULL argument

dwl.c

@@ -480,9 +480,9 @@ arrange(Monitor *m)
 	wlr_scene_node_set_enabled(&m->fullscreen_bg->node,
 			(c = focustop(m)) && c->isfullscreen);
 
-	if (m)
-		strncpy(m->ltsymbol, m->lt[m->sellt]->symbol, LENGTH(m->ltsymbol));
-	if (m && m->lt[m->sellt]->arrange)
+	strncpy(m->ltsymbol, m->lt[m->sellt]->symbol, LENGTH(m->ltsymbol));
+
+	if (m->lt[m->sellt]->arrange)
 		m->lt[m->sellt]->arrange(m);
 	motionnotify(0);
 	checkidleinhibitor(NULL);