netdev: netdev_get_etheraddr is not functioning as advertised.

netdev_get_etheraddr claims to clear 'mac' on error, but it fails to do so.
When looking further into both netdev_windows_get_etheraddr() and
netdev_linux_get_etheraddr(), 'mac' is also not cleared. This will lead to
usage of uninitialised ofputil_phy_port.hw_addr.

v1 -> v2: fixed a bug in v1 found by Ben, thanks Ben.

Signed-off-by: Yifeng Sun <pkusunyifeng@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>

lib/netdev.c

@@ -852,7 +852,13 @@ netdev_set_etheraddr(struct netdev *netdev, const struct eth_addr mac)
 int
 netdev_get_etheraddr(const struct netdev *netdev, struct eth_addr *mac)
 {
-    return netdev->netdev_class->get_etheraddr(netdev, mac);
+    int error;
+
+    error = netdev->netdev_class->get_etheraddr(netdev, mac);
+    if (error) {
+        memset(mac, 0, sizeof *mac);
+    }
+    return error;
 }
 
 /* Returns the name of the network device that 'netdev' represents,