pgsql: Avoid an Assert failure in deconstruct_array() by making

Log Message:
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Avoid an Assert failure in deconstruct_array() by making get_attstatsslot()
use the actual element type of the array it's disassembling, rather than
trusting the type OID passed in by its caller. This is needed because
sometimes the planner passes in a type OID that's only binary-compatible
with the target column's type, rather than being an exact match. Per an
example from Bernd Helmle.

Possibly we should refactor get_attstatsslot/free_attstatsslot to not expect
the caller to supply type ID data at all, but for now I'll just do the
minimum-change fix.

Back-patch to 7.4. Bernd's test case only crashes back to 8.0, but since
these subroutines are the same in 7.4, I suspect there may be variant
cases that would crash 7.4 as well.

Tags:
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REL8_1_STABLE

Modified Files:
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pgsql/src/backend/utils/cache:
lsyscache.c (r1.129.2.1 -> r1.129.2.2)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/cache/lsyscache.c?r1=1.129.2.1&r2=1.129.2.2)