verify c->mon in setfullscreen()

try to open a client in fullscreen mode (example: foot -F) causes that
dwl crash, this prevent crash but isn't a solution, since client will
be open in normal mode, we must find a way to set c->mon in createnotify
function

dwl.c

@@ -1018,6 +1018,9 @@ togglefullscreen(const Arg *arg)
 void
 setfullscreen(Client *c, int fullscreen)
 {
+	if (!c->mon)
+		return;
+
 	c->isfullscreen = fullscreen;
 	c->bw = (1 - fullscreen) * borderpx;
 	client_set_fullscreen(c, fullscreen);