ret may be > 0 on success path at this point. Fix assertion, which may
crash currently.

Fixes: 4ce5dd3e
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20200526181347.489557-1-vsementsov@virtuozzo.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>