ath10k: make sure to not use invalid beacon pointer

If DMA mapping of next beacon failed it was
possible for next SWBA to access a pointer that
was already unmapped and freed. This could cause
memory corruption.

Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>

drivers/net/wireless/ath/ath10k/wmi.c

@@ -1431,6 +1431,7 @@ static void ath10k_wmi_event_host_swba(struct ath10k *ar, struct sk_buff *skb)
 					 ATH10K_SKB_CB(arvif->beacon)->paddr,
 					 arvif->beacon->len, DMA_TO_DEVICE);
 			dev_kfree_skb_any(arvif->beacon);
+			arvif->beacon = NULL;
 		}
 
 		ATH10K_SKB_CB(bcn)->paddr = dma_map_single(arvif->ar->dev,