pgsql: Fix pgstat_report_waiting() to not dump core if called before

Log Message:
-----------
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.

Modified Files:
--------------
    pgsql/src/backend/postmaster:
        pgstat.c (r1.137 -> r1.138)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/pgstat.c.diff?r1=1.137&r2=1.138)