pgsql: Fix overly tense optimization of PLpgSQL_func_hashkey: we must

Log Message:
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Fix overly tense optimization of PLpgSQL_func_hashkey: we must represent
the isTrigger state explicitly, not rely on nonzero-ness of trigrelOid
to indicate trigger-hood, because trigrelOid will be left zero when compiling
for validation. The (useless) function hash entry built by the validator
was able to match an ordinary non-trigger call later in the same session,
thereby bypassing the check that is supposed to prevent such a call.
Per report from Alvaro.

It might be worth suppressing the useless hash entry altogether, but
that's a bigger change than I want to consider back-patching.

Back-patch to 8.0. 7.4 doesn't have the problem because it doesn't
have validation mode.

Tags:
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REL8_1_STABLE

Modified Files:
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pgsql/src/pl/plpgsql/src:
pl_comp.c (r1.94.2.4 -> r1.94.2.5)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/pl_comp.c?r1=1.94.2.4&r2=1.94.2.5)
plpgsql.h (r1.65.2.3 -> r1.65.2.4)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/plpgsql.h?r1=1.65.2.3&r2=1.65.2.4)