network: always initialize struct nl_handler

network: always initialize struct nl_handler

Despite the fact that struct nl_handler is filled zeros
in netlink_open() there are two cases where we have possible
exit paths from the function before netlink_open() is called.

At the same time we have cleaner registered:
call_cleaner(netlink_close)

Two cases:
- netdev_get_flag
- lxc_ipvlan_create

If we are exiting from these functions before netlink_open()
is called we will close random file descriptor by reading
it from (struct nl_handler)->fd.

Let's just properly initialize this structure in all cases
to prevent this bug in the future.

Reported-by: coverity (CID #1517319 and #1517316)
Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>