Correcting the ACM upmixing from mono/stereo to surround

This CL corrects the upmixing from mono/stereo to surround in the audio
coding module.


Bug: webrtc:11083
Change-Id: Ic529107d59ff54a8e48b0424cbdf2b49b7a65c12
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/159705
Commit-Queue: Per Åhgren <peah@webrtc.org>
Reviewed-by: Sam Zackrisson <saza@webrtc.org>
Reviewed-by: Henrik Lundin <henrik.lundin@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29792}

modules/audio_coding/acm2/audio_coding_module.cc

diff --git a/modules/audio_coding/acm2/audio_coding_module.cc b/modules/audio_coding/acm2/audio_coding_module.cc
index 314afd7..0232f27 100644
--- a/modules/audio_coding/acm2/audio_coding_module.cc
+++ b/modules/audio_coding/acm2/audio_coding_module.cc
@@ -246,18 +246,22 @@
   size_t in_index = 0;
   size_t out_index = 0;
 
-  // When upmixing is needed, duplicate the last channel of the input.
+  // When upmixing is needed, copy the available channels directly, and set the
+  // remaining channels to zero.
   if (input.num_channels_ < num_output_channels) {
     for (size_t k = 0; k < input.samples_per_channel_; ++k) {
       for (size_t j = 0; j < input.num_channels_; ++j) {
         (*output)[out_index++] = input_data[in_index++];
       }
-      RTC_DCHECK_GT(in_index, 0);
-      const int16_t value_last_channel = input_data[in_index - 1];
       for (size_t j = input.num_channels_; j < num_output_channels; ++j) {
-        (*output)[out_index++] = value_last_channel;
+        (*output)[out_index++] = 0;
       }
+      RTC_DCHECK_EQ(in_index, (k + 1) * input.num_channels_);
+      RTC_DCHECK_EQ(out_index, (k + 1) * num_output_channels);
     }
+    RTC_DCHECK_EQ(in_index, input.samples_per_channel_ * input.num_channels_);
+    RTC_DCHECK_EQ(out_index, input.samples_per_channel_ * num_output_channels);
+
     return;
   }