Division by zero in RNN-VAD.

Bug: webrtc:9450, chromium:861557
Change-Id: I00ddda1fe0e088b983707420acf1b9a6763a3535
Reviewed-on: https://webrtc-review.googlesource.com/87841
Reviewed-by: Alessio Bazzica <alessiob@webrtc.org>
Commit-Queue: Alex Loiko <aleloi@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23999}

modules/audio_processing/agc2/rnn_vad/lp_residual.cc

@@ -12,6 +12,7 @@
 
 #include <algorithm>
 #include <array>
+#include <cmath>
 #include <numeric>
 
 #include "rtc_base/checks.h"
@@ -61,6 +62,13 @@ void ComputeInitialInverseFilterCoefficients(
       reflection_coeff += lpc_coeffs[j] * auto_corr[i - j];
     }
     reflection_coeff += auto_corr[i + 1];
+
+    // Avoid division by numbers close to zero.
+    constexpr float kMinErrorMagnitude = 1e-6f;
+    if (std::fabs(error) < kMinErrorMagnitude) {
+      error = std::copysign(kMinErrorMagnitude, error);
+    }
+
     reflection_coeff /= -error;
     // Update LPC coefficients and total error.
     lpc_coeffs[i] = reflection_coeff;