Adds fuzzer for the residual echo detector.

webrtc/modules/audio_processing/echo_detector/normalized_covariance_estimator.cc

Index: webrtc/modules/audio_processing/echo_detector/normalized_covariance_estimator.cc
diff --git a/webrtc/modules/audio_processing/echo_detector/normalized_covariance_estimator.cc b/webrtc/modules/audio_processing/echo_detector/normalized_covariance_estimator.cc
index 0b8b68bfbdd0a247851e145e9bdee7078cb84a1f..8408f014048205e7f982dfe3bd7f2162d37ec087 100644
--- a/webrtc/modules/audio_processing/echo_detector/normalized_covariance_estimator.cc
+++ b/webrtc/modules/audio_processing/echo_detector/normalized_covariance_estimator.cc
@@ -10,6 +10,10 @@
 
 #include "webrtc/modules/audio_processing/echo_detector/normalized_covariance_estimator.h"
 
+#include <math.h>
+
+#include "webrtc/base/checks.h"
+
 namespace webrtc {
 namespace {
 
@@ -27,6 +31,8 @@ void NormalizedCovarianceEstimator::Update(float x,
   covariance_ =
       (1.f - kAlpha) * covariance_ + kAlpha * (x - x_mean) * (y - y_mean);
   normalized_cross_correlation_ = covariance_ / (x_sigma * y_sigma + .0001f);
+  RTC_DCHECK(isfinite(covariance_));
+  RTC_DCHECK(isfinite(normalized_cross_correlation_));
 }
 
 void NormalizedCovarianceEstimator::Clear() {