Merge pull request #828 from jitsi/this_stop_undefined

fix(RTPStatsCollector): this.stop is 'undefined'

modules/statistics/RTPStatsCollector.js

                         self.baselineAudioLevelsReport
                             = self.currentAudioLevelsReport;
                     },
-                    self.errorCallback
+                    error => self.errorCallback(error)
                 );
             },
             self.audioLevelsIntervalMilis
 
                         self.previousStatsReport = self.currentStatsReport;
                     },
-                    self.errorCallback
+                    error => self.errorCallback(error)
                 );
             },
             self.statsIntervalMilis