prevent an infinite loop if try to use focusmon() with all monitors disabled

dwl.c

@@ -1192,9 +1192,11 @@ focusclient(Client *c, int lift)
 void
 focusmon(const Arg *arg)
 {
-	do
+	int i = 0, nmons = wl_list_length(&mons);
+	if (nmons)
+		do /* don't switch to disabled mons */
 			selmon = dirtomon(arg->i);
-	while (!selmon->wlr_output->enabled);
+		while (!selmon->wlr_output->enabled && i++ < nmons);
 	focusclient(focustop(selmon), 1);
 }