bugfix/dtls-crash-onclose

[OctopusRage]

• the strange behaviour occured randomly when my peer tried to close its connection
• this fix dtls_transport = nil causing peer connection crashes when closed
• already try catch PeerConnection.close(). but still cannot avoid this error because its nif implementation (?)
• this pr eliminate my current workaround by checking PeerConnection.get_dtls_transport_state(peer_connection) not in [:failed, :closed] before closing peer connection