pgsql: Fix mistaken Assert in adjust_appendrel_attr_needed, per Greg

Log Message:
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Fix mistaken Assert in adjust_appendrel_attr_needed, per Greg Stark.

Tags:
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REL8_2_STABLE

Modified Files:
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pgsql/src/backend/optimizer/prep:
prepunion.c (r1.134 -> r1.134.2.1)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/prep/prepunion.c.diff?r1=1.134&r2=1.134.2.1)