If the peer offers mppe stateful mode (H bit clear), accept it if

mppe-stateful was an option.

This bug wasn't discovered earlier because I haven't seen a system
that offers stateful mode "only".  A bug report from Dmitry Glushenok
shows that Windows RRAS refuses stateless mode for dialup connections.
For dialup, it almost makes sense that one could use stateful mode safely,
but if you accept that argument (difficult to attack a dialup connection)
then it also follows that mppe as a whole is not useful over dialup.