ath9k: fix double-free bug on beacon generate failure

When the next beacon is sent, the ath_buf from the previous run is reused.
If getting a new beacon from mac80211 fails, bf->bf_mpdu is not reset, yet
the skb is freed, leading to a double-free on the next beacon tx attempt,
resulting in a system crash.

Cc: stable@vger.kernel.org
Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

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

@@ -147,6 +147,7 @@ static struct ath_buf *ath9k_beacon_generate(struct ieee80211_hw *hw,
 				 skb->len, DMA_TO_DEVICE);
 		dev_kfree_skb_any(skb);
 		bf->bf_buf_addr = 0;
+		bf->bf_mpdu = NULL;
 	}
 
 	skb = ieee80211_beacon_get(hw, vif);