session_disconnect: clear state
session_disconnect: clear state

If authentication is started but not completed before the application
gives up and instead wants to shut down the session, the '->state' field
might still be set and thus effectively dead-lock session_disconnect.

This happens because both _libssh2_transport_send() and
_libssh2_transport_read() refuse to do anything as long as state is set
without the LIBSSH2_STATE_KEX_ACTIVE bit.

Reported in curl bug https://github.com/curl/curl/issues/3650

Closes #310

libssh2/libssh2,libssh2/libssh2,libssh2/libssh2,libssh2/libssh2