mac80211: fix tx select key null pointer crash with hostapd

Pavel Roskin reported a crash in ieee80211_tx_h_select_key():

http://marc.info/?l=linux-wireless&m=126419655108528&w=2

This is a regression from patch "mac80211: move control.hw_key assignment".
Fix it as suggested by Johannes, adding an else statement to make sure
that tx->key is not accessed when it's null.

Compile-tested only.
Reported-by: Pavel Roskin <proski@gnu.org>
Cc: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Kalle Valo <kalle.valo@iki.fi>
Tested-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

net/mac80211/tx.c

@@ -547,9 +547,10 @@ ieee80211_tx_h_select_key(struct ieee80211_tx_data *tx)
 			    !ieee80211_use_mfp(hdr->frame_control, tx->sta,
 					       tx->skb))
 				tx->key = NULL;
-			skip_hw = (tx->key->conf.flags &
-						IEEE80211_KEY_FLAG_SW_MGMT) &&
-				   ieee80211_is_mgmt(hdr->frame_control);
+			else
+				skip_hw = (tx->key->conf.flags &
+					   IEEE80211_KEY_FLAG_SW_MGMT) &&
+					ieee80211_is_mgmt(hdr->frame_control);
 			break;
 		case ALG_AES_CMAC:
 			if (!ieee80211_is_mgmt(hdr->frame_control))