don't access sel in restack without checking for NULL (multihead crashing bug)

view.c

@@ -207,6 +207,7 @@ restack()
 
 	fi = 0;
 	mi = 2 * f;
+	if(sel) {
 		if(sel->isfloat || arrange == dofloat) {
 			wins[fi++] = sel->twin;
 			wins[fi++] = sel->win;
@@ -215,6 +216,7 @@ restack()
 			wins[mi++] = sel->twin;
 			wins[mi++] = sel->win;
 		}
+	}
 	for(c = clients; c; c = c->next)
 		if(isvisible(c) && c != sel) {
 			if(c->isfloat || arrange == dofloat) {