pgsql: Fix overly-strict sanity check in BeginInternalSubTransaction

[tgl(at)postgresql(dot)org (Tom Lane)]

Log Message:
-----------
Fix overly-strict sanity check in BeginInternalSubTransaction that made it
fail when used in a deferred trigger.  Bug goes back to 8.0; no doubt the
reason it hadn't been noticed is that we've been discouraging use of
user-defined constraint triggers.  Per report from Frank van Vugt.

Tags:
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REL8_2_STABLE

Modified Files:
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    pgsql/src/backend/access/transam:
        xact.c (r1.229.2.1 -> r1.229.2.2)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xact.c.diff?r1=1.229.2.1&r2=1.229.2.2)