Fix div-by-0 in NetEq's StatisticsCalculator

If a StatisticsCalculator::PeriodicUmaAverage object was created and
then deleted without any samples being logged, the destructor would call
the Metric() method, which calculated sum_/counter_. However, with no
samples logged, counter_ is 0.

This was found and verified using UBSan tests; see the bug for more info.

BUG=webrtc:5490
R=ivoc@webrtc.org

Review URL: https://codereview.webrtc.org/1678773003

Cr-Commit-Position: refs/heads/master@{#11534}

webrtc/modules/audio_coding/neteq/statistics_calculator.cc

diff --git a/webrtc/modules/audio_coding/neteq/statistics_calculator.cc b/webrtc/modules/audio_coding/neteq/statistics_calculator.cc
index 8f87376..88b7710 100644
--- a/webrtc/modules/audio_coding/neteq/statistics_calculator.cc
+++ b/webrtc/modules/audio_coding/neteq/statistics_calculator.cc
@@ -95,7 +95,7 @@
 }
 
 int StatisticsCalculator::PeriodicUmaAverage::Metric() const {
-  return static_cast<int>(sum_ / counter_);
+  return counter_ == 0 ? 0 : static_cast<int>(sum_ / counter_);
 }
 
 void StatisticsCalculator::PeriodicUmaAverage::Reset() {