Fix iterator use. The iterator now iterates over the BUN and tail values.  This means we can't just use the first (i.e. BUN) value as if it was the head of the iterator bat.  We need to fetch the value from the bat to get the head value.
Fix iterator use.
The iterator now iterates over the BUN and tail values.  This means we
can't just use the first (i.e. BUN) value as if it was the head of the
iterator bat.  We need to fetch the value from the bat to get the head
value.

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