applied Hiltjo's tiny cleanup fix

dwm.c

@@ -1270,7 +1270,7 @@ propertynotify(XEvent *e) {
 		default: break;
 		case XA_WM_TRANSIENT_FOR:
 			if(!c->isfloating && (XGetTransientForHint(dpy, c->win, &trans)) &&
-			   (c->isfloating = (wintoclient(trans))))
+			   (c->isfloating = (wintoclient(trans)) != NULL))
 				arrange(c->mon);
 			break;
 		case XA_WM_NORMAL_HINTS: