wimax: wimax_msg_alloc() returns ERR_PTR not null

wimax_msg_alloc() returns an ERR_PTR and not null.  I changed it to test
for ERR_PTR instead of null.  I also added a check in front of the
kfree() because kfree() can handle null but not ERR_PTR.
Signed-off-by: Dan Carpenter <error27@gmail.com>

drivers/net/wimax/i2400m/rx.c

@@ -300,17 +300,16 @@ void i2400m_rx_ctl_ack(struct i2400m *i2400m,
 		d_printf(1, dev, "Huh? waiter for command reply cancelled\n");
 		goto error_waiter_cancelled;
 	}
-	if (ack_skb == NULL) {
+	if (IS_ERR(ack_skb))
 		dev_err(dev, "CMD/GET/SET ack: cannot allocate SKB\n");
-		i2400m->ack_skb = ERR_PTR(-ENOMEM);
-	} else
-		i2400m->ack_skb = ack_skb;
+	i2400m->ack_skb = ack_skb;
 	spin_unlock_irqrestore(&i2400m->rx_lock, flags);
 	complete(&i2400m->msg_completion);
 	return;
 
 error_waiter_cancelled:
-	kfree_skb(ack_skb);
+	if (!IS_ERR(ack_skb))
+		kfree_skb(ack_skb);
 error_no_waiter:
 	spin_unlock_irqrestore(&i2400m->rx_lock, flags);
 	return;