net/eth.c: throw BUG for eth_get_dev_by_name(NULL)

eth_get_dev_by_name() is not safe to use for devname being NULL
as it uses strcmp. This patch makes it fail with a BUG().

Signed-off-by: Helmut Raiger <helmut.raiger@hale.at>
Acked-by: Mike Frysinger <vapier@gentoo.org>

net/eth.c

@@ -108,6 +108,8 @@ struct eth_device *eth_get_dev_by_name(const char *devname)
 {
 	struct eth_device *dev, *target_dev;
 
+	BUG_ON(devname == NULL);
+
 	if (!eth_devices)
 		return NULL;