dhparam_cb not unset on destruction

[daurnimator]

I was reading through the code, and saw that the callback dhparam_cb is setup via SSL_CTX_set_tmp_dh_callback, but not torn down.
This means if the reference to the ssl context is collected while a DH negotiation is pending, there will be an illegal de-reference (segfault if we're lucky) in dhparam_cb. I don't think this can currently be reached within the confines of luasec, but if a SSL_CTX* or SSL* ever escaped then there would be a problem.
For safety, meth_destroy should unset the dh callback.