codec/rtcp/client_test.go: TestReceiveAndSend occasionally blocks and results in a failed test.

I believe this is caused by the fact that we make the Client.err chan in the Client.Start func, but that means that if we have some sort of error checking routine that is started before Client.Start() is called, then the call to Client.Err() will block. A fix is simply to initialise the err chan in the constructor.