ath10k: Check tx_stats before use it

tx_stats will be freed and set to NULL before debugfs_sta node is
removed in station disconnetion process. So if read the debugfs_sta
node there may be NULL pointer error. Add check for tx_stats before
use it to resove this issue.
Signed-off-by: Yingying Tang <yintang@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>

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

@@ -663,6 +663,13 @@ static ssize_t ath10k_dbg_sta_dump_tx_stats(struct file *file,
 
 	mutex_lock(&ar->conf_mutex);
 
+	if (!arsta->tx_stats) {
+		ath10k_warn(ar, "failed to get tx stats");
+		mutex_unlock(&ar->conf_mutex);
+		kfree(buf);
+		return 0;
+	}
+
 	spin_lock_bh(&ar->data_lock);
 	for (k = 0; k < ATH10K_STATS_TYPE_MAX; k++) {
 		for (j = 0; j < ATH10K_COUNTER_TYPE_MAX; j++) {