wifi: mac80211: fix potential null pointer dereference

rx->sta->amsdu_mesh_control is being passed to ieee80211_amsdu_to_8023s
without checking rx->sta. Since it doesn't make sense to accept A-MSDU
packets without a sta, simply add a check earlier.

Fixes: 6e4c0d04

 ("wifi: mac80211: add a workaround for receiving non-standard mesh A-MSDU")
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Link: https://lore.kernel.org/r/20230330090001.60750-2-nbd@nbd.name
Signed-off-by: Johannes Berg <johannes.berg@intel.com>