Due to it in contrast to the decoder not setting up low_delay,
the code in parse_nal_units would always end up setting has_b_frames
to 1 (except when stream is explicitly marked as low delay).
Since the parser itself would create extradata, simply reopening
the parser would cause this.
estimate_timings_from_pts would cause the parser to be reopened
on the same stream.

This fixes trac issue #360.
Signed-off-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>