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

Log Message:
-----------
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:
----
REL8_0_STABLE

Modified Files:
--------------
    pgsql/src/backend/utils/cache:
        lsyscache.c (r1.119.4.1 -> r1.119.4.2)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/cache/lsyscache.c?r1=1.119.4.1&r2=1.119.4.2)