[TG3]: Fix possible NULL deref in tg3_run_loopback().

tg3_run_loopback doesn't check that dev_alloc_skb() returns anything
useful.

Even if dev_alloc_skb() fails to return an skb to us we'll happily go
on and assume it did, so we risk dereferencing a NULL pointer. Much
better to fail gracefully by returning -ENOMEM than crashing here.

Signed-off-by: Jesper Juhl <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/tg3.c

···
 
 	tx_len = 1514;
 	skb = dev_alloc_skb(tx_len);
+	if (!skb)
+		return -ENOMEM;
+
 	tx_data = skb_put(skb, tx_len);
 	memcpy(tx_data, tp->dev->dev_addr, 6);
 	memset(tx_data + 6, 0x0, 8);