applied Martin Hurton's checkotherwm simplification

dwm.c

@@ -337,15 +337,14 @@ buttonpress(XEvent *e) {
 void
 checkotherwm(void) {
 	otherwm = False;
-	XSetErrorHandler(xerrorstart);
+	xerrorxlib = XSetErrorHandler(xerrorstart);
 
 	/* this causes an error if some other window manager is running */
 	XSelectInput(dpy, DefaultRootWindow(dpy), SubstructureRedirectMask);
 	XSync(dpy, False);
 	if(otherwm)
 		die("dwm: another window manager is already running\n");
-	XSetErrorHandler(NULL);
-	xerrorxlib = XSetErrorHandler(xerror);
+	XSetErrorHandler(xerror);
 	XSync(dpy, False);
 }