Cuts imported from a flac file have a random/null end point

Defines RDWaveFile ext_time_length/time_length for flac format.

**Before**

![rivendell-wavefile-fix-flac-timelength-before](https://cloud.githubusercontent.com/assets/24570/4515500/d638cbba-4bc0-11e4-83dc-2532f178f0d2.png)

**After**

![rivendell-wavefile-fix-flac-timelength](https://cloud.githubusercontent.com/assets/24570/4515501/d64504d4-4bc0-11e4-9900-9ada47127bb9.png)

Thank to Hervé from Radio Grésivaudan for the report