pgsql: Fix AfterTriggerSaveEvent to use a test and elog, not just

Log Message:
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Fix AfterTriggerSaveEvent to use a test and elog, not just Assert, to check
that it's called within an AfterTriggerBeginQuery/AfterTriggerEndQuery pair.
The RI cascade triggers suppress that overhead on the assumption that they
are always run non-deferred, so it's possible to violate the condition if
someone mistakenly changes pg_trigger to mark such a trigger deferred.
We don't really care about supporting that, but throwing an error instead
of crashing seems desirable.  Per report from Marcelo Costa.

Modified Files:
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    pgsql/src/backend/commands:
        trigger.c (r1.255 -> r1.256)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/trigger.c?r1=1.255&r2=1.256)