libbinder: Return error instead of crashing on closed Trusty connection

If RpcServerTrusty rejects a client connection, it still receives
a few more messages. Since the connection was rejected, the internal
channel has not been initialized. Handle this corner case by returning
an ERR_BAD_STATE error code instead of a fatal crash so that Trusty
services do not crash after rejecting connections.

Bug: 242940548
Test: Presubmit
Change-Id: Ibb1445c26616a8c7e369d1fd7a3fb3602d6f2155