mac80211: add missing key check

ieee80211_tx_h_select_key might decide that a frame
need not be encrypted at all, in which case it will
clear tx->key. In that case it may crash if a key
was previously selected, e.g. as the default key.

This is also due to my patch
"mac80211: move control.hw_key assignment".

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

net/mac80211/tx.c

@@ -557,7 +557,7 @@ ieee80211_tx_h_select_key(struct ieee80211_tx_data *tx)
 			break;
 		}
 
-		if (!skip_hw &&
+		if (!skip_hw && tx->key &&
 		    tx->key->conf.flags & KEY_FLAG_UPLOADED_TO_HARDWARE)
 			info->control.hw_key = &tx->key->conf;
 	}