decoder/ffmpeg: work around bogus channel count

Initialize the audio_format before calling avcodec_open(), because
avcodec_open() will fill bogus values.

NEWS

@@ -1,6 +1,7 @@
 ver 0.16.6 (2010/??/??)
 * decoder:
   - fix assertion failure when resuming streams
+  - ffmpeg: work around bogus channel count
 * encoder:
   - flac, null, wave: fix buffer corruption bug
   - wave: support packed 24 bit samples

src/decoder/ffmpeg_decoder_plugin.c

@@ -407,13 +407,6 @@ ffmpeg_decode(struct decoder *decoder, struct input_stream *input)
 		return;
 	}
 
-	if (avcodec_open(codec_context, codec)<0) {
-		g_warning("Could not open codec\n");
-		av_close_input_stream(format_context);
-		mpd_ffmpeg_stream_close(stream);
-		return;
-	}
-
 	GError *error = NULL;
 	struct audio_format audio_format;
 	if (!audio_format_init_checked(&audio_format,
@@ -422,7 +415,18 @@ ffmpeg_decode(struct decoder *decoder, struct input_stream *input)
 				       codec_context->channels, &error)) {
 		g_warning("%s", error->message);
 		g_error_free(error);
-		avcodec_close(codec_context);
+		av_close_input_stream(format_context);
+		mpd_ffmpeg_stream_close(stream);
+		return;
+	}
+
+	/* the audio format must be read from AVCodecContext by now,
+	   because avcodec_open() has been demonstrated to fill bogus
+	   values into AVCodecContext.channels - a change that will be
+	   reverted later by avcodec_decode_audio3() */
+
+	if (avcodec_open(codec_context, codec)<0) {
+		g_warning("Could not open codec\n");
 		av_close_input_stream(format_context);
 		mpd_ffmpeg_stream_close(stream);
 		return;