WebTorrent: Prevent protocol encryption over WebRTC

[paullouisageneau]

This PR fixes the assert m_recv_buffer.packet_size() == dh_key_len from aux::bt_peer_connection::on_receive_impl in #5100

WebTorrent connections are always made over DTLS and therefore should never use protocol encryption.

For reference: #4123

[arvidn]

do you think the issue happened when libtorrent attempted to connect to Webtorrent peers that don't support protocol encryption?
ideally this assert wouldn't fire even in that case though, but if that's the problem, it's probably unrelated to webtorrent support anyway.

[paullouisageneau]

Yes, this is the situation that seems to trigger the issue. It's not perfectly clear to me what happens but PE should definitely not be used for WebTorrent, and the Javascript client doesn't support it.