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>