mac80211: fix an oops in several failure paths in key allocation

This patch fixes an oops in several failure paths in key allocation. This
Oops occurs when freeing a key that has not been linked yet, so the
key->sdata is not set.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>