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

src/backend/executor/execPartition.c

@@ -87,6 +87,7 @@ ExecSetupPartitionTupleRouting(ModifyTableState *mtstate,
 		num_update_rri = list_length(node->plans);
 		proute->subplan_partition_offsets =
 			palloc(num_update_rri * sizeof(int));
+		proute->num_subplan_partition_offsets = num_update_rri;
 
 		/*
 		 * We need an additional tuple slot for storing transient tuples that
@@ -481,6 +482,7 @@ ExecCleanupTupleRouting(PartitionTupleRouting *proute)
 		 * result rels are present in the UPDATE subplans.
 		 */
 		if (proute->subplan_partition_offsets &&
+			subplan_index < proute->num_subplan_partition_offsets &&
 			proute->subplan_partition_offsets[subplan_index] == i)
 		{
 			subplan_index++;

src/backend/executor/nodeModifyTable.c

@@ -1812,7 +1812,8 @@ tupconv_map_for_subplan(ModifyTableState *mtstate, int whichplan)
 		 * If subplan-indexed array is NULL, things should have been arranged
 		 * to convert the subplan index to partition index.
 		 */
-		Assert(proute && proute->subplan_partition_offsets != NULL);
+		Assert(proute && proute->subplan_partition_offsets != NULL &&
+			   whichplan < proute->num_subplan_partition_offsets);
 
 		leaf_index = proute->subplan_partition_offsets[whichplan];
 

src/include/executor/execPartition.h

@@ -80,6 +80,7 @@ typedef struct PartitionDispatchData *PartitionDispatch;
  * subplan_partition_offsets	Integer array ordered by UPDATE subplans. Each
  *								element of this array has the index into the
  *								corresponding partition in partitions array.
+ * num_subplan_partition_offsets  Length of 'subplan_partition_offsets' array
  * partition_tuple_slot			TupleTableSlot to be used to manipulate any
  *								given leaf partition's rowtype after that
  *								partition is chosen for insertion by
@@ -96,6 +97,7 @@ typedef struct PartitionTupleRouting
 	TupleConversionMap **child_parent_tupconv_maps;
 	bool	   *child_parent_map_not_required;
 	int		   *subplan_partition_offsets;
+	int			num_subplan_partition_offsets;
 	TupleTableSlot *partition_tuple_slot;
 	TupleTableSlot *root_tuple_slot;
 } PartitionTupleRouting;