pulsesink: Uncork stream while flushing the ringbuffer

pulsesink: Uncork stream while flushing the ringbuffer

After starting the ringbuffer, we wait for enough data to arrive before
uncorking the stream. This will cause the pipeline to stall if we get an
EOS (or otherwise need to flush the stream) before sufficient data
becomes available. This patch makes sure that the stream is uncorked
while flushing to avoid this problem.

Fixes issue with a webkit unit test testing reverse playback of
an MP4 H.264/AAC file.

https://bugzilla.gnome.org/show_bug.cgi?id=639740