The executing code had a safety assertion so that it refused to free Items
that it didn't create.  However, there is a case, undefined user variables,
which would put Items into the list to be freed.

Instead, do something that is more risky in expectation that the code will 
be refactored soon, as Kostja wants to do:  Remove the assertions from 
prepare() and execute().  Put one assertion at a higher level, before 
stmt->set_params_from_vars(), which may then create new to-be-freed Items .