Fix get_object_namespace() not to think extensions are "in" a schema.

extnamespace means something altogether different in this context.
Mostly by accident, this coding error (introduced in my commit
82a4a777d9) broke the buildfarm instead
of just silently doing the wrong thing.

src/backend/catalog/objectaddress.c

@@ -116,7 +116,7 @@ static ObjectPropertyType ObjectProperty[] =
 		ExtensionRelationId,
 		ExtensionOidIndexId,
 		-1,
-		Anum_pg_extension_extnamespace
+		InvalidAttrNumber		/* extension doesn't belong to extnamespace */
 	},
 	{
 		ForeignDataWrapperRelationId,