In beacon protection, don't leave skb->next/prev pointing to the
on-stack list, even if that's actually harmless since we don't use
them again afterwards.

While at it, check that the SKB on the list is still the same, as
that's required here. If not, the encryption (protection) code is
buggy.

Fixes: 0a3a8436 ("mac80211: Beacon protection using the new BIGTK (AP)")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Link: https://lore.kernel.org/r/20200320102021.1be7823fc05e.Ia89fb79a0469d32137c9a04315a1d2dfc7b7d6f5@changeidSigned-off-by: Johannes Berg <johannes.berg@intel.com>