wifi: mt76: connac: check for null before dereferencing

The wcid can be NULL. It should be checked for validity before
dereferencing it to avoid crash.

Fixes: 098428c4 ("wifi: mt76: connac: set correct muar_idx for mt799x chipsets")
Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>

drivers/net/wireless/mediatek/mt76/mt76_connac_mcu.c

@@ -283,7 +283,7 @@ __mt76_connac_mcu_alloc_sta_req(struct mt76_dev *dev, struct mt76_vif *mvif,
 	};
 	struct sk_buff *skb;
 
-	if (is_mt799x(dev) && !wcid->sta)
+	if (is_mt799x(dev) && wcid && !wcid->sta)
 		hdr.muar_idx = 0xe;
 
 	mt76_connac_mcu_get_wlan_idx(dev, wcid, &hdr.wlan_idx_lo,