Fix bug in join pushdown (where joins may have been pushed down without proper subtree marks).
Fix bug in join pushdown
(where joins may have been pushed down without proper subtree marks).

zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb