tstimeshifter: Added support for buffering query - post-review fix-ups

- Checking for non-NULL index in refactored
  gst_ts_seeker_bytes_to_stream_time()

- Checking if the buffering query is an upstream query
  For this moment there's no indication of other use cases

- Added comment to make it clear that a buffering query handled in tsseeker
  needs to have time format.  Otherwise it's simply forwarded upstream.

- Fixed white space consistency problems in video-player.py