Metadata graph enumeration can throw from a number
functions and break things in horrible ways.
Here we sanitize against the most egregious offenders,
but not all possible sources.

Change-Id: I40e006ea433dd7274d4fa08f3e8f8507680ef2f4
Reviewed-on: https://gerrit.libreoffice.org/63009
Tested-by: Jenkins
Reviewed-by: Ashod Nakashian <ashnakash@gmail.com>