Fix a bug in vad.reset() (#2023)

We also need to clear _last

@@ -126,6 +126,7 @@ class VoiceActivityDetector::Impl {
 
 
     model_->Reset();
     model_->Reset();
     buffer_.Reset();
     buffer_.Reset();
+    last_.clear();
 
 
     start_ = -1;
     start_ = -1;
   }
   }