Make sure we don't assert when calling fileStatus

Make sure we don't assert when calling fileStatus

It happens that sometimes we leak a directory path ending with a slash,
but that violates fileStatus' precondition so let's catch it early and
skip such path.

Of course the right fix would be a larger swipe in the sync engine and
around it to not use naked strings anymore but rely on the typesystem.

Signed-off-by: Kevin Ottens <kevin.ottens@nextcloud.com>