mount: fix segmentation fault in can_mount_now on no mnt_master

In resolve_shared_mounts there are cases when we have m->master_id > 0
but m->mnt_master is not set, it happens for cases where we have no
access to master mount, for instance CT root m->parent==NULL or when
mount is external. In can_mount_now  for such master mounts we don't
need to check mounted state also, so just use "if(mi->mnt_master)"
condition instead of "if(mi->master_id > 0)" to fix segfault.

https://jira.sw.ru/browse/PSBM-86978



Program terminated with signal 11, Segmentation fault.
0x000000000046328b in can_mount_now (mi=0x2155970) at criu/mount.c:2699
2699	list_for_each_entry(c, &mi->mnt_master->children, siblings)

(gdb) p mi->mnt_master
$2 = (struct mount_info	*) 0x0

Fixes commit 3a02362c5be1 ("mount: fix can_mount_now to wait children of
master's share properly")

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