wimax: checking ERR_PTR vs null

stch_skb is allocated with wimax_gnl_re_state_change_alloc().  That
function returns ERR_PTRs on failure and doesn't return NULL.
Signed-off-by: NDan Carpenter <error27@gmail.com>

net/wimax/stack.c

@@ -315,7 +315,7 @@ void __wimax_state_change(struct wimax_dev *wimax_dev, enum wimax_st new_state)
 		BUG();
 	}
 	__wimax_state_set(wimax_dev, new_state);
-	if (stch_skb)
+	if (!IS_ERR(stch_skb))
 		wimax_gnl_re_state_change_send(wimax_dev, stch_skb, header);
 out:
 	d_fnend(3, dev, "(wimax_dev %p new_state %u [old %u]) = void\n",