ath9k: fix a null pointer deref issue

SVN-Revision: 24312

package/mac80211/patches/320-pending_work.patch

@@ -2137,3 +2137,17 @@
  	}
  
  	/* add interfaces */
+--- a/drivers/net/wireless/ath/ath9k/xmit.c
++++ b/drivers/net/wireless/ath/ath9k/xmit.c
+@@ -1745,7 +1745,10 @@ int ath_tx_start(struct ieee80211_hw *hw
+ 	int frmlen = skb->len + FCS_LEN;
+ 	int q;
+ 
+-	txctl->an = (struct ath_node *)sta->drv_priv;
++	/* NOTE:  sta can be NULL according to net/mac80211.h */
++	if (sta)
++		txctl->an = (struct ath_node *)sta->drv_priv;
++
+ 	if (info->control.hw_key)
+ 		frmlen += info->control.hw_key->icv_len;
+