channeledtransport: fix segfault during callback destruction

onDestroy/onRecv should not be called after destruction. Because
ChanneledSIPTransport is managed by PJSIP, we can't pass a weak_ptr to the
callbacks, so just replace the callbacks during the destruction.

Change-Id: Ibc3f2baf8b4cdcfe45652fd14d1acc9d38a07af4