pgsql: Fix another case in which autovacuum would fail while analyzing

Log Message:
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Fix another case in which autovacuum would fail while analyzing
expressional indexes. Per report from Brian Hirt.

Tags:
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REL8_1_STABLE

Modified Files:
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pgsql/src/backend/commands:
vacuum.c (r1.317.2.1 -> r1.317.2.2)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/vacuum.c.diff?r1=1.317.2.1&r2=1.317.2.2)