Remove NULL dereference from RenameRelationInternal().

Defect in last week's commit aac2c9b4fd,
per Coverity.  Reaching this would need catalog corruption.  Back-patch
to v12, like that commit.

src/backend/commands/tablecmds.c

@@ -4132,9 +4132,9 @@ RenameRelationInternal(Oid myrelid, const char *newrelname, bool is_internal, bo
 	relrelation = table_open(RelationRelationId, RowExclusiveLock);
 
 	reltup = SearchSysCacheLockedCopy1(RELOID, ObjectIdGetDatum(myrelid));
-	otid = reltup->t_self;
 	if (!HeapTupleIsValid(reltup))	/* shouldn't happen */
 		elog(ERROR, "cache lookup failed for relation %u", myrelid);
+	otid = reltup->t_self;
 	relform = (Form_pg_class) GETSTRUCT(reltup);
 
 	if (get_relname_relid(newrelname, namespaceId) != InvalidOid)