pgsql: Make plan_cluster_use_sort cope with no IndexOptInfo for the tar

Make plan_cluster_use_sort cope with no IndexOptInfo for the target index.

The original coding assumed that such a case represents caller error, but
actually get_relation_info will omit generating an IndexOptInfo for any
index it thinks is unsafe to use. Therefore, handle this case by returning
"true" to indicate that a seqscan-and-sort is the preferred way to
implement the CLUSTER operation. New bug in 9.1, no backpatch needed.
Per bug #5985 from Daniel Grace.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/5b8e442953da0bf4950b86c7cb4a6117842aedf7

Modified Files
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src/backend/optimizer/plan/planner.c | 29 ++++++++++++++++++-----------
1 files changed, 18 insertions(+), 11 deletions(-)