Fix little dropout when transition to next track. Updates #4542.

SourceDrainedCallback() has a valid next url and calls TransitionToNext(). This sets a new decode bin for the pipeline. The pipeline then buffers the new track and sends GST_MESSAGE_BUFFERING. BufferingMessageReceived() then pauses the pipeline because it's buffering. But we are buffering the next track and not the current one, so the pipeline has still data and doesn't need to be paused.