Check disk stats before throwing on open

Quick fix to do a last-chance update_stats_from_disc when trying to open a file that has $self->e == 0.  This solves an issue where a file (here a fofn) had the wrong file permissions when the db entry was created (so had e==0), but fixing the file permissions did not fix the open as the database was out of sync with the file on disc.

This should have no side-effects, as the next statement would have thrown an error anyway.