Enforce child constraints during COPY TO a partitioned table.

The previous coding inadvertently checked the constraints for the
partitioned table rather than the target partition, which could
lead to data in a partition that fails to satisfy some constraint
on that partition.  This problem seems to date back to when
table partitioning was introduced; prior to that, there was only
one target table for a COPY, so the problem didn't occur, and the
code just didn't get updated.

Etsuro Fujita, reviewed by Amit Langote and Ashutosh Bapat

Discussion: https://postgr.es/message-id/5ABA4074.1090500%40lab.ntt.co.jp

src/backend/commands/copy.c

@@ -2709,7 +2709,8 @@ CopyFrom(CopyState cstate)
 					check_partition_constr = false;
 
 				/* Check the constraints of the tuple */
-				if (cstate->rel->rd_att->constr || check_partition_constr)
+				if (resultRelInfo->ri_RelationDesc->rd_att->constr ||
+					check_partition_constr)
 					ExecConstraints(resultRelInfo, slot, estate, true);
 
 				if (useHeapMultiInsert)