Fixing warning C4267 on Win (more_configs).

This is a follow-up of https://webrtc-review.googlesource.com/c/src/+/12921.

Bug: chromium:759980
Change-Id: Ifd39adb6541c0c7e0337f587a8b34b84a07331ed
Reviewed-on: https://webrtc-review.googlesource.com/13122
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20341}

modules/audio_coding/neteq/audio_multi_vector_unittest.cc

diff --git a/modules/audio_coding/neteq/audio_multi_vector_unittest.cc b/modules/audio_coding/neteq/audio_multi_vector_unittest.cc
index 051b8ee..1489e80 100644
--- a/modules/audio_coding/neteq/audio_multi_vector_unittest.cc
+++ b/modules/audio_coding/neteq/audio_multi_vector_unittest.cc
@@ -9,6 +9,7 @@
  */
 
 #include "modules/audio_coding/neteq/audio_multi_vector.h"
+#include "rtc_base/safe_conversions.h"
 
 #include <assert.h>
 #include <stdlib.h>
@@ -51,7 +52,7 @@
     // And so on.
     for (size_t i = 0; i < array_length(); ++i) {
       for (size_t j = 1; j <= num_channels_; ++j) {
-        *ptr = j * 100 + i;
+        *ptr = rtc::checked_cast<int16_t>(j * 100 + i);
         ++ptr;
       }
     }