wifi: rtl8xxxu: check vif before using in rtl8xxxu_tx()

The 'vif' is from tx_info of SKB, and other codes check 'vif' before using,
which raises smatch warnings:

drivers/net/wireless/realtek/rtl8xxxu/rtl8xxxu_core.c:5656 rtl8xxxu_tx()
   warn: variable dereferenced before check 'vif' (see line 5553)

Compile tested only.
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://msgid.link/20240216033923.34683-1-pkshih@realtek.com

drivers/net/wireless/realtek/rtl8xxxu/rtl8xxxu_core.c

@@ -5550,7 +5550,7 @@ static void rtl8xxxu_tx(struct ieee80211_hw *hw,
 	struct rtl8xxxu_tx_urb *tx_urb;
 	struct ieee80211_sta *sta = NULL;
 	struct ieee80211_vif *vif = tx_info->control.vif;
-	struct rtl8xxxu_vif *rtlvif = (struct rtl8xxxu_vif *)vif->drv_priv;
+	struct rtl8xxxu_vif *rtlvif = vif ? (struct rtl8xxxu_vif *)vif->drv_priv : NULL;
 	struct device *dev = &priv->udev->dev;
 	u32 queue, rts_rate;
 	u16 pktlen = skb->len;
@@ -5621,7 +5621,7 @@ static void rtl8xxxu_tx(struct ieee80211_hw *hw,
 		default:
 			break;
 		}
-		if (bmc && rtlvif->hw_key_idx != 0xff) {
+		if (bmc && rtlvif && rtlvif->hw_key_idx != 0xff) {
 			tx_desc->txdw1 |= cpu_to_le32(TXDESC_EN_DESC_ID);
 			macid = rtlvif->hw_key_idx;
 		}