mac80211: fix possible NULL pointer dereference

This patch moves 'key' dereference after BUG_ON(!key) so that when key is NULL
we will see proper trace instead of oops.

Signed-off-by: Mariusz Kozlowski <mk@lab.zgora.pl>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>