JAQLparser: fix crashes on subsequent (wrong) statements
JAQLparser: fix crashes on subsequent (wrong) statements

Make sure we initialised the parse-tree to NULL.
Read away any statements left in the buffer for as long as we don't
really cope well with multistatements.

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