baseparse: Don't set meaningless buffer dts from segment->start
When we do not have any information about DTSs we shouldn't try to make
them up, moreover after seeking segment->start
has nothing to do with
the next buffer timing (and is probably after the actual buffer timestamp)
and since, since fa831247
we do:
if (buffer->dts > buffer->dts)
buffer->pts = buffer->dts
we end up setting buffer->pts = segment->start
which is plain
broken and leads to downstream decoder accept the first buffer
as it will be inside the segment (its pts==segment->start) which
basically means accurate seeking behaves mostly the same way as
keyframe seeks.
Fixes #492 (closed)