mount: fix uninitialized use of fd on switch_ns error

close_safe can operate uninitialized fd in case of error in switch_ns,
found by Coverity Scan:

*** CID 187164:  Uninitialized variables  (UNINIT)
/criu/mount.c: 1313 in open_mountpoint()
1307     err:
1308            return 1;
1309     }
1310
1311     int open_mountpoint(struct mount_info *pm)
1312     {
>>>     CID 187164:  Uninitialized variables  (UNINIT)
>>>     Declaring variable "fd" without initializer.
1313            int fd, cwd_fd, ns_old = -1;
1314
1315            /* No overmounts and children - the entire mount is visible */
1316            if (list_empty(&pm->children) && !mnt_is_overmounted(pm))
1317                    return __open_mountpoint(pm, -1);
1318

Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>