mac80211: fix access to null skb

Without this patch, if xmit_skb is null but net_ratelimit() returns 0 we would
go to the else branch and access the null xmit_skb. Pointed out by Johannes
Berg.

Signed-off-by: Luis Carlos Cobo <luisca@cozybit.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>