Fix usage of palloc() in MERGE/SPLIT PARTITION(s) code

f2e4cc4279 and 4b3d173629 implement ALTER TABLE ... MERGE/SPLIT
PARTITION(s) commands.  In several places, these commits use palloc(),
where we should use palloc_object() and palloc_array().  This commit
provides appropriate usage of palloc_object() and palloc_array().

Reported-by: Man Zeng <zengman@halodbtech.com>
Discussion: https://postgr.es/m/tencent_3661BB522D5466B33EA33666%40qq.com

src/backend/commands/tablecmds.c

@@ -22317,7 +22317,7 @@ createTableConstraints(List **wqueue, AlteredTableInfo *tab,
 			 */
 			if (attribute->attgenerated == ATTRIBUTE_GENERATED_STORED)
 			{
-				newval = (NewColumnValue *) palloc0(sizeof(NewColumnValue));
+				newval = palloc0_object(NewColumnValue);
 				newval->attnum = num;
 				newval->expr = expression_planner((Expr *) def);
 				newval->is_generated = (attribute->attgenerated != '\0');
@@ -22406,7 +22406,7 @@ createTableConstraints(List **wqueue, AlteredTableInfo *tab,
 			{
 				NewConstraint *newcon;
 
-				newcon = (NewConstraint *) palloc0(sizeof(NewConstraint));
+				newcon = palloc0_object(NewConstraint);
 				newcon->name = ccon->name;
 				newcon->contype = CONSTR_CHECK;
 				newcon->qual = qual;
@@ -22944,7 +22944,7 @@ createSplitPartitionContext(Relation partRel)
 {
 	SplitPartitionContext *pc;
 
-	pc = (SplitPartitionContext *) palloc0(sizeof(SplitPartitionContext));
+	pc = palloc0_object(SplitPartitionContext);
 	pc->partRel = partRel;
 
 	/*

src/backend/partitioning/partbounds.c

@@ -5115,8 +5115,7 @@ calculate_partition_bound_for_merge(Relation parent,
 				int			nparts = list_length(partOids);
 				List	   *bounds = NIL;
 
-				lower_bounds = (PartitionRangeBound **)
-					palloc0(nparts * sizeof(PartitionRangeBound *));
+				lower_bounds = palloc0_array(PartitionRangeBound *, nparts);
 
 				/*
 				 * Create an array of lower bounds and a list of
@@ -5755,8 +5754,7 @@ check_partitions_for_split(Relation parent,
 	 * Make an array new_parts with new partitions except the DEFAULT
 	 * partition.
 	 */
-	new_parts = (SinglePartitionSpec **)
-		palloc0(list_length(partlist) * sizeof(SinglePartitionSpec *));
+	new_parts = palloc0_array(SinglePartitionSpec *, list_length(partlist));
 
 	/* isSplitPartDefault flag: is split partition a DEFAULT partition? */
 	isSplitPartDefault = (defaultPartOid == splitPartOid);
@@ -5786,8 +5784,7 @@ check_partitions_for_split(Relation parent,
 		 * all partitions in ascending order of their bounds (we compare the
 		 * lower bound only).
 		 */
-		lower_bounds = (PartitionRangeBound **)
-			palloc0(nparts * sizeof(PartitionRangeBound *));
+		lower_bounds = palloc0_array(PartitionRangeBound *, nparts);
 
 		/* Create an array of lower bounds. */
 		for (i = 0; i < nparts; i++)
@@ -5802,8 +5799,7 @@ check_partitions_for_split(Relation parent,
 
 		/* Reorder the array of partitions. */
 		tmp_new_parts = new_parts;
-		new_parts = (SinglePartitionSpec **)
-			palloc0(nparts * sizeof(SinglePartitionSpec *));
+		new_parts = palloc0_array(SinglePartitionSpec *, nparts);
 		for (i = 0; i < nparts; i++)
 			new_parts[i] = tmp_new_parts[lower_bounds[i]->index];