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>