netdev: netdev_get_etheraddr is not functioning as advertised.

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>