brcm80211: brcmu_pkt_buf_free_skb() should handle NULL

This is potentially called with NULL pointers, for example, look at
brcmf_c_prec_enq().  Since it's a free() function, probably people
expect it to handle NULL pointers.
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/brcm80211/brcmutil/utils.c

@@ -43,6 +43,8 @@ EXPORT_SYMBOL(brcmu_pkt_buf_get_skb);
 /* Free the driver packet. Free the tag if present */
 void brcmu_pkt_buf_free_skb(struct sk_buff *skb)
 {
+	if (!skb)
+		return;
 	WARN_ON(skb->next);
 	if (skb->destructor)
 		/* cannot kfree_skb() on hard IRQ (net/core/skbuff.c) if