pgsql: Avoid referencing off the end of subplan_partition_offsets.

Avoid referencing off the end of subplan_partition_offsets.

Report by buildfarm member skink and Tom Lane.  Analysis by me.
Patch by Amit Khandekar.

Discussion: http://postgr.es/m/CAJ3gD9fVA1iXQYhfqHP5n_TEd4U9=V8TL_cc-oKRnRmxgdvJrQ@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/945f71db845262e7491b5fe4403b01147027576b

Modified Files
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src/backend/executor/execPartition.c   | 2 ++
src/backend/executor/nodeModifyTable.c | 3 ++-
src/include/executor/execPartition.h   | 2 ++
3 files changed, 6 insertions(+), 1 deletion(-)