Fix pgstat_report_waiting() to not dump core if called before

Fix pgstat_report_waiting() to not dump core if called before
pgstat_bestart() has been called; else any lock-block occurring
during InitPostgres() is disastrous.  I believe this explains
recent wasp regression failure; at least it explains the crash I
got while trying to duplicate the problem.  I also made
pgstat_report_activity() safe against the same scenario, just
in case.  The report_waiting hazard was created by my patch of
19-Aug to include waiting status in pg_stat_activity.