mt76: fix memcpy to potential null pointer on failed allocation

Currently if the allocation of skb fails and returns NULL then the
call to skb_put will cause a null pointer dereference. Fix this by
checking for a null skb and returning NULL.  Note that calls to
function mt76x2_mcu_msg_alloc don't directly check the null return
but instead pass the NULL pointer to mt76x2_mcu_msg_send which
checks for the NULL and returns ENOMEM in this case.

Detected by CoverityScan, CID#1462624 ("Dereference null return value")

Fixes: 7bc04215a6 ("mt76: add driver code for MT76x2e")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>

drivers/net/wireless/mediatek/mt76/mt76x2_mcu.c

@@ -45,6 +45,8 @@ static struct sk_buff *mt76x2_mcu_msg_alloc(const void *data, int len)
 	struct sk_buff *skb;
 
 	skb = alloc_skb(len, GFP_KERNEL);
+	if (!skb)
+		return NULL;
 	memcpy(skb_put(skb, len), data, len);
 
 	return skb;