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: 6e4c0d0460 ("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>

net/mac80211/rx.c

@@ -2930,7 +2930,7 @@ __ieee80211_rx_h_amsdu(struct ieee80211_rx_data *rx, u8 data_offset)
 					  data_offset, true))
 		return RX_DROP_UNUSABLE;
 
-	if (rx->sta && rx->sta->amsdu_mesh_control < 0) {
+	if (rx->sta->amsdu_mesh_control < 0) {
 		bool valid_std = ieee80211_is_valid_amsdu(skb, true);
 		bool valid_nonstd = ieee80211_is_valid_amsdu(skb, false);
 
@@ -3006,7 +3006,7 @@ ieee80211_rx_h_amsdu(struct ieee80211_rx_data *rx)
 		}
 	}
 
-	if (is_multicast_ether_addr(hdr->addr1))
+	if (is_multicast_ether_addr(hdr->addr1) || !rx->sta)
 		return RX_DROP_UNUSABLE;
 
 	if (rx->key) {