rtlwifi: Fix kernel oops introduced with commit e49656147359

With commit e49656147359 {"rtlwifi: Use dev_kfree_skb_irq instead of
kfree_skb"), the method used to free an skb was changed because the
kfree_skb() was inside a spinlock. What was forgotten is that kfree_skb()
guards against a NULL value for the argument. Routine dev_kfree_skb_irq()
does not, and a test is needed to prevent kernel panics.

Fixes: e49656147359 ("rtlwifi: Use dev_kfree_skb_irq instead of kfree_skb")
Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Cc: Stable <stable@vger.kernel.org> # 4.9+
Cc: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>

drivers/net/wireless/realtek/rtlwifi/core.c

@@ -1829,7 +1829,8 @@ bool rtl_cmd_send_packet(struct ieee80211_hw *hw, struct sk_buff *skb)
 
 	spin_lock_irqsave(&rtlpriv->locks.irq_th_lock, flags);
 	pskb = __skb_dequeue(&ring->queue);
-	dev_kfree_skb_irq(pskb);
+	if (pskb)
+		dev_kfree_skb_irq(pskb);
 
 	/*this is wrong, fill_tx_cmddesc needs update*/
 	pdesc = &ring->desc[0];