mac80211: fix a potential NULL access in ieee80211_crypto_hw_decrypt

The NULL pointer access could happen when ieee80211_crypto_hw_decrypt
is called from ieee80211_rx_h_decrypt with the following condition:
1. rx->key->conf.cipher is not WEP, CCMP, TKIP or AES_CMAC
2. rx->sta is NULL

When ieee80211_crypto_hw_decrypt is called, it verifies
rx->sta->cipher_scheme and it will cause Oops if rx->sta is NULL.

This path adds an addirional rx->sta == NULL verification in
ieee80211_crypto_hw_decrypt for this case.
Signed-off-by: Max Stepanov <Max.Stepanov@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/wpa.c

@@ -811,7 +811,7 @@ ieee80211_crypto_hw_encrypt(struct ieee80211_tx_data *tx)
 ieee80211_rx_result
 ieee80211_crypto_hw_decrypt(struct ieee80211_rx_data *rx)
 {
-	if (rx->sta->cipher_scheme)
+	if (rx->sta && rx->sta->cipher_scheme)
 		return ieee80211_crypto_cs_decrypt(rx);
 
 	return RX_DROP_UNUSABLE;