fix assert placement in get_request_id
fix assert placement in get_request_id

The old behavior could lead to cases where highest_request_id could be
incremented beyond max_request_id and not fixed.

mambocab/python-driver,datastax/python-driver,thelastpickle/python-driver,thelastpickle/python-driver,mambocab/python-driver,datastax/python-driver