Fix corruption of tableElts list by MergeAttributes().

Since commit e7b3349a, MergeAttributes
destructively modifies the input List, to which the caller's
CreateStmt still points.  One may wonder whether this was already a
bug, but commit f0e44751 made things
noticeably worse by adding additional destructive modifications so
that the caller's List might, in the case of creation a partitioned
table, no longer even be structurally valid.  Restore the status quo
ante by assigning the return value of MergeAttributes back to
stmt->tableElts in the caller.

In most of the places where DefineRelation is called, it doesn't
matter what stmt->tableElts points to here or whether it's valid or
not, because the caller doesn't use the statement for anything after
DefineRelation returns anyway.  However, ProcessUtilitySlow passes it
to EventTriggerCollectSimpleCommand, and that function tries to invoke
copyObject on it.  If any of the CreateStmt's substructure is invalid
at that point, undefined behavior will result.

One might wonder whether this whole area needs further revision -
perhaps DefineRelation() ought not to be destructively modifying the
caller-provided CreateStmt at all.  However, that would be a behavior
change for any event triggers using C code to inspect the CreateStmt,
so for now, just fix the crash.

Report by Amit Langote, who provided a somewhat different patch for it.

Discussion: http://postgr.es/m/bf6a39a7-100a-74bd-1156-3c16a1429d88@lab.ntt.co.jp

src/backend/commands/tablecmds.c

@@ -491,7 +491,6 @@ DefineRelation(CreateStmt *stmt, char relkind, Oid ownerId,
 {
 	char		relname[NAMEDATALEN];
 	Oid			namespaceId;
-	List	   *schema = stmt->tableElts;
 	Oid			relationId;
 	Oid			tablespaceId;
 	Relation	rel;
@@ -614,9 +613,11 @@ DefineRelation(CreateStmt *stmt, char relkind, Oid ownerId,
 
 	/*
 	 * Look up inheritance ancestors and generate relation schema, including
-	 * inherited attributes.
+	 * inherited attributes.  (Note that stmt->tableElts is destructively
+	 * modified by MergeAttributes.)
 	 */
-	schema = MergeAttributes(schema, stmt->inhRelations,
+	stmt->tableElts =
+		MergeAttributes(stmt->tableElts, stmt->inhRelations,
 						stmt->relation->relpersistence,
 						stmt->partbound != NULL,
 						&inheritOids, &old_constraints, &parentOidCount);
@@ -626,7 +627,7 @@ DefineRelation(CreateStmt *stmt, char relkind, Oid ownerId,
 	 * deals with column names, types, and NOT NULL constraints, but not
 	 * default values or CHECK constraints; we handle those below.
 	 */
-	descriptor = BuildDescForRelation(schema);
+	descriptor = BuildDescForRelation(stmt->tableElts);
 
 	/*
 	 * Notice that we allow OIDs here only for plain tables and partitioned
@@ -667,7 +668,7 @@ DefineRelation(CreateStmt *stmt, char relkind, Oid ownerId,
 	cookedDefaults = NIL;
 	attnum = 0;
 
-	foreach(listptr, schema)
+	foreach(listptr, stmt->tableElts)
 	{
 		ColumnDef  *colDef = lfirst(listptr);