Zero initialize OPUS stream mapping data.

On parse failure, FFmpeg does not always error out, so ensure the OPUS
initializer we construct is completely initialized.

BUG=389280
TEST=none

Review URL: https://codereview.chromium.org/382123002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@282731 0039d316-1c4b-4281-b951-d872f2087c98

media/filters/opus_audio_decoder.cc

@@ -173,7 +173,8 @@ struct OpusExtraData {
         channel_mapping(0),
         num_streams(0),
         num_coupled(0),
-        gain_db(0) {
+        gain_db(0),
+        stream_map() {
     memcpy(stream_map,
            kDefaultOpusChannelLayout,
            kMaxChannelsWithDefaultLayout);