Fix clusterfuzz DCHECK

Calling avformat_find_stream_info can change the number of streams in
the AVFormatContext. This means that an index that we thought was
pointing towards an audio stream can now be referring to a video stream
after the call.

This CL fixes the issue by finding a new audio stream after calling
avformat_find_stream_info.

Bug: 1025964
Change-Id: I81f661412dcb108865925a37cb254db89938d00f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1953395
Commit-Queue: Thomas Guilbert <tguilbert@chromium.org>
Auto-Submit: Thomas Guilbert <tguilbert@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#722724}

media/filters/audio_file_reader.cc

@@ -51,7 +51,15 @@ bool AudioFileReader::OpenDemuxer() {
     return false;
   }
 
-  // Find the first audio stream, if any.
+  const int result = avformat_find_stream_info(format_context, NULL);
+  if (result < 0) {
+    DLOG(WARNING)
+        << "AudioFileReader::Open() : error in avformat_find_stream_info()";
+    return false;
+  }
+
+  // Calling avformat_find_stream_info can uncover new streams. We wait till now
+  // to find the first audio stream, if any.
   codec_context_.reset();
   bool found_stream = false;
   for (size_t i = 0; i < format_context->nb_streams; ++i) {
@@ -66,13 +74,6 @@ bool AudioFileReader::OpenDemuxer() {
   if (!found_stream)
     return false;
 
-  const int result = avformat_find_stream_info(format_context, NULL);
-  if (result < 0) {
-    DLOG(WARNING)
-        << "AudioFileReader::Open() : error in avformat_find_stream_info()";
-    return false;
-  }
-
   // Get the codec context.
   codec_context_ =
       AVStreamToAVCodecContext(format_context->streams[stream_index_]);