default to Dir=/ in dpkg/status file finding magic

default to Dir=/ in dpkg/status file finding magic

Seen in cme #833656 if Dir isn't set (yet) we end up later absoluting a
path which was supposed to be absolute already, so if Dir is empty we
assume it to be '/' instead. In practice this is a bug in the software
using libapt, but for maxium compatibility lets explicitly set the
default value here to be safe.

Reported-By: Paul Wise <pabs@debian.org>
Inspired-By: Brendan O'Dea <bod@debian.org>
Fixes-Regression: 475f75506db48a7fa90711fce4ed129f6a14cc9a
Shadows-Bug: #833656