Fix use-after-free introduced in 55ed3def

Evidenced by failure under RELCACHE_FORCE_RELEASE (buildfarm member
prion).

Author: Amit Langote
Discussion: https://postgr.es/m/CA+HiwqGV=k_Eh4jBiQw66ivvdG+EUkrEYeHTYL1SvDj_YOYV0g@mail.gmail.com

src/backend/commands/indexcmds.c

@@ -1059,13 +1059,11 @@ DefineIndex(Oid relationId,
 
 			memcpy(part_oids, partdesc->oids, sizeof(Oid) * nparts);
 
-			parentDesc = CreateTupleDescCopy(RelationGetDescr(rel));
+			parentDesc = RelationGetDescr(rel);
 			opfamOids = palloc(sizeof(Oid) * numberOfKeyAttributes);
 			for (i = 0; i < numberOfKeyAttributes; i++)
 				opfamOids[i] = get_opclass_family(classObjectId[i]);
 
-			table_close(rel, NoLock);
-
 			/*
 			 * For each partition, scan all existing indexes; if one matches
 			 * our index definition and is not already attached to some other
@@ -1265,13 +1263,12 @@ DefineIndex(Oid relationId,
 				heap_freetuple(newtup);
 			}
 		}
-		else
-			table_close(rel, NoLock);
 
 		/*
 		 * Indexes on partitioned tables are not themselves built, so we're
 		 * done here.
 		 */
+		table_close(rel, NoLock);
 		if (!OidIsValid(parentIndexId))
 			pgstat_progress_end_command();
 		return address;