linux-user: NETLINK_LIST_MEMBERSHIPS: Allow bad ptr if its length is 0

linux-user: NETLINK_LIST_MEMBERSHIPS: Allow bad ptr if its length is 0

getsockopt(fd, SOL_NETLINK, NETLINK_LIST_MEMBERSHIPS, *optval, *optlen)
syscall allows optval to be NULL/invalid if optlen points to a size of
zero. This allows userspace to query the length of the array they should
use to get the full membership list before allocating memory for said
list, then re-calling getsockopt with proper optval/optlen arguments.

Notable users of this pattern include systemd-networkd, which in the
(albeit old) version 237 tested, cannot start without this fix.

Signed-off-by: Frédéric Fortier <frf@ghgsat.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20210328180135.88449-1-frf@ghgsat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>