adm_config_report: improve invalid config handling

This is a follow up on https://github.com/mantisbt/mantisbt/pull/224#issuecomment-51663947, as the pull request was merged and closed just as I was writing ;-)

I'm proposing a less intrusive/more user-friendly way of handling an invalid config_id by silently resetting $t_filter_config_value = META_FILTER_NONE, similar to what is done with $t_filter_project_value.

As a side note, should we not also do the same for an invalid user ?