FIX: work harder to find a valid duration for the music stream

Use xbmc.Player() duration information if a valid duration is not found in the MusicInfoTag

* NOTE: This change allows scrobbling of many more audio streams that may not be
  able to calculate & set a duration prior to handing off to the xbmc.Player

@ronie, this change to the libre.fm scrobbler is similar to the change that I just requested for the last.fm scrobbler.

Thanks for your attention.