NET: Fix system hanging if NET device is not installed

If we try to boot from NET device, NetInitLoop in net.c will be invoked. If NET device is not installed, eth_get_dev() function will return eth_current value, which is NULL.
When NetInitLoop is called, "eth_get_dev->enetaddr" will access restricted memory area and therefore cause hanging.
This issue is found on Tegra30 Cardhu platform after adding CONFIG_CMD_NET and CONFIG_CMD_DHCP in config header file.

---
 net/net.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)

index df94789..7663b9c 100644
@@ -271,7 +271,8 @@ static void NetInitLoop(void)
 #endif
 		env_changed_id = env_id;
 	}
-	memcpy(NetOurEther, eth_get_dev()->enetaddr, 6);
+	if (eth_get_dev())
+		memcpy(NetOurEther, eth_get_dev()->enetaddr, 6);
 
 	return;
 }

1.7.7

Oh, you didn't send this patch to anyone, just the mailing list. I added the net maintainer to Cc so he'll see this patch.

BTW, this is a critical bugfix, to avoid hangs without any USB Ethernet device attached.