[scrobbles] Fix if past seconds is zero

vrobbler/apps/scrobbles/models.py

@@ -1246,6 +1246,7 @@ class Scrobble(TimeStampedModel):
 
         # Set our playback seconds, and calc long play seconds
         self.playback_position_seconds = seconds_elapsed
+        past_seconds = 0
         if self.previous:
             past_seconds = self.previous.long_play_seconds