Unlike filesrc, flacenc outputs the flac blocks neatly aligned one in
each buffer. This means that when we switch from metadata mode to
audio data passthrough mode, there's no data left in the adapter to
push out at this point, so check if there's data in the adapter
before requesting buffers from it (also needed in case we get input
buffers of 0 size).
Fixes#615793.
1351af141d