Adds fuzzer for the residual echo detector.

webrtc/modules/audio_processing/echo_detector/mean_variance_estimator.cc

Index: webrtc/modules/audio_processing/echo_detector/mean_variance_estimator.cc
diff --git a/webrtc/modules/audio_processing/echo_detector/mean_variance_estimator.cc b/webrtc/modules/audio_processing/echo_detector/mean_variance_estimator.cc
index 06baf68b8a72ccd5d00118871a175b58a2585412..f0de0f09fe4d0f3e7492ee974531f3e5fc14fd68 100644
--- a/webrtc/modules/audio_processing/echo_detector/mean_variance_estimator.cc
+++ b/webrtc/modules/audio_processing/echo_detector/mean_variance_estimator.cc
@@ -26,6 +26,8 @@ void MeanVarianceEstimator::Update(float value) {
   mean_ = (1.f - kAlpha) * mean_ + kAlpha * value;
   variance_ =
       (1.f - kAlpha) * variance_ + kAlpha * (value - mean_) * (value - mean_);
+  RTC_DCHECK(isfinite(mean_));
+  RTC_DCHECK(isfinite(variance_));
 }
 
 float MeanVarianceEstimator::std_deviation() const {