pgsql: Relax an Assert() that has been found to be too strict in some

Log Message:
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Relax an Assert() that has been found to be too strict in some situations
involving unions of types having typmods.  Variants of the failure are known
to occur in 8.1 and up; not sure if it's possible in 8.0 and 7.4, but since
the code exists that far back, I'll just patch 'em all.  Per report from
Brian Hurt.

Tags:
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REL8_1_STABLE

Modified Files:
--------------
    pgsql/src/backend/executor:
        execScan.c (r1.37 -> r1.37.2.1)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/execScan.c.diff?r1=1.37&r2=1.37.2.1)