Commented by: @hvlad

In the call's sequence above all thee isc_dsql_XXX calls was defered and executed (sent over the wire) only when isc_start_transaction was called. But isc_dsql_free_statement with DSQL_drop option already deleted statement (while added op_free_statement packet to the deferred packets queue). Therefore when responce on op_execute was handled statement was not exists and isc_bad_req_handle error is raised.

The fix delayed statement deletion until handling of responce on op_free_statement packet