Query prepare & bind for qmark crashes

[gijzelaerr]

When running branch #110

and executing this code a couple of times we sometimes get a crash:

import monetdbe

con1 = monetdbe.connect()
con1.execute("create table test(i int, s text)")
con1.execute("insert into test(i, s) values (?, ?)", (5, "bla"))

con2 = monetdbe.connect()
con2.execute("create table test(i int, s text)")
con2.execute("insert into test(i, s) values (?, ?)", (5, "bla"))

con1.execute("insert into test(i, s) values (?, ?)", (5, "bla"))

Using Oct2020 (not Oct2020-sp1)