avformat/riffdec: don't unconditionally overwrite WAVEFORMATEXTENSIBLE layout

Do it only if the value conflicts with the previous channels value.

Fixes ticket #9912

Signed-off-by: James Almer <jamrial@gmail.com>
(cherry picked from commit 60d8c201)

libavformat/riffdec.c

@@ -102,6 +102,8 @@ int ff_get_wav_header(AVFormatContext *s, AVIOContext *pb,
         return AVERROR_INVALIDDATA;
     }
 
+    av_channel_layout_uninit(&par->ch_layout);
+
     par->codec_type  = AVMEDIA_TYPE_AUDIO;
     if (!big_endian) {
         id                 = avio_rl16(pb);
@@ -189,9 +191,12 @@ int ff_get_wav_header(AVFormatContext *s, AVIOContext *pb,
     if (par->codec_id == AV_CODEC_ID_ADPCM_G726 && par->sample_rate)
         par->bits_per_coded_sample = par->bit_rate / par->sample_rate;
 
-    av_channel_layout_uninit(&par->ch_layout);
-    par->ch_layout.order       = AV_CHANNEL_ORDER_UNSPEC;
-    par->ch_layout.nb_channels = channels;
+    /* ignore WAVEFORMATEXTENSIBLE layout if different from channel count */
+    if (channels != par->ch_layout.nb_channels) {
+        av_channel_layout_uninit(&par->ch_layout);
+        par->ch_layout.order       = AV_CHANNEL_ORDER_UNSPEC;
+        par->ch_layout.nb_channels = channels;
+    }
 
     return 0;
 }