rtlwifi: Fix kernel oops introduced with commit e4965614

With commit e4965614 {"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: e4965614 ("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>