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>
diff --git a/net/eth.c b/net/eth.c
index dbd1e2d..5911b1c 100644
--- a/net/eth.c
+++ b/net/eth.c
@@ -108,6 +108,8 @@
 {
 	struct eth_device *dev, *target_dev;
 
+	BUG_ON(devname == NULL);
+
 	if (!eth_devices)
 		return NULL;