ath9k: Check for NULL sta in ath_tx_start

It can be NULL according to docs, and logging showed it
to be NULL in practice.
Signed-off-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/ath/ath9k/xmit.c

@@ -1740,7 +1740,10 @@ int ath_tx_start(struct ieee80211_hw *hw, struct sk_buff *skb,
 	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;