[ffmpeg_audio_decoder] Bail out on unknown sample format.

Crashes suggest that we're sometimes passing out an AudioBuffer which
has an unknown sample format. This shouldn't be allowed.

BUG=1032338
TEST=none, can't reproduce.
R=tguilbert

Change-Id: I2562e8c60a9666e91a2003c5272d600474720ae9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1988891
Commit-Queue: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: Thomas Guilbert <tguilbert@chromium.org>
Auto-Submit: Dale Curtis <dalecurtis@chromium.org>
Reviewed-by: Thomas Guilbert <tguilbert@chromium.org>
Cr-Commit-Position: refs/heads/master@{#728766}

media/base/audio_buffer.cc

@@ -82,6 +82,8 @@ AudioBuffer::AudioBuffer(SampleFormat sample_format,
   if (!create_buffer)
     return;
 
+  CHECK_NE(sample_format, kUnknownSampleFormat);
+
   int data_size_per_channel = frame_count * bytes_per_channel;
   if (IsPlanar(sample_format)) {
     DCHECK(!IsBitstreamFormat()) << sample_format_;

media/filters/ffmpeg_audio_decoder.cc

@@ -369,6 +369,11 @@ int FFmpegAudioDecoder::GetAudioBuffer(struct AVCodecContext* s,
   AVSampleFormat format = static_cast<AVSampleFormat>(frame->format);
   SampleFormat sample_format =
       AVSampleFormatToSampleFormat(format, s->codec_id);
+  if (sample_format == kUnknownSampleFormat) {
+    DLOG(ERROR) << "Unknown sample format: " << format;
+    return AVERROR(EINVAL);
+  }
+
   int channels = DetermineChannels(frame);
   if (channels <= 0 || channels >= limits::kMaxChannels) {
     DLOG(ERROR) << "Requested number of channels (" << channels