Modify sequence catalog tuple before invoking post alter hook.

This seems to have been broken in the commit (1753b1b0) that
moved the sequence definition into pg_sequence.

Author: Andres Freund
Discussion: https://postgr.es/m/20170601000716.qxg7c46ukkiljjb3@alap3.anarazel.de
Backpatch: Bug is in master/v10 only

src/backend/commands/sequence.c

@@ -490,12 +490,12 @@ AlterSequence(ParseState *pstate, AlterSeqStmt *stmt)
 	if (owned_by)
 		process_owned_by(seqrel, owned_by, stmt->for_identity);
 
+	CatalogTupleUpdate(rel, &seqtuple->t_self, seqtuple);
+
 	InvokeObjectPostAlterHook(RelationRelationId, relid, 0);
 
 	ObjectAddressSet(address, RelationRelationId, relid);
 
-	CatalogTupleUpdate(rel, &seqtuple->t_self, seqtuple);
-
 	heap_close(rel, RowExclusiveLock);
 	relation_close(seqrel, NoLock);