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.

Modified Files:
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pgsql/src/backend/commands:
vacuum.c (r1.319 -> r1.320)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/vacuum.c.diff?r1=1.319&r2=1.320)