files: Handle the absence of ID in proc fdinfo

This makes crtools correctly abort when working on wrong kernel.
Otherwise all the open files will have the same (garbage) ID and
the subsequent restore will result in broken app.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>