Do not set mkv bit_depth to av_get_bytes_per_sample() for G.726.

The value is wrong and leads to broken files.

libavformat/matroskaenc.c

@@ -620,7 +620,7 @@ static int mkv_write_tracks(AVFormatContext *s)
             continue;
         }
 
-        if (!bit_depth)
+        if (!bit_depth && codec->codec_id != AV_CODEC_ID_ADPCM_G726)
             bit_depth = av_get_bytes_per_sample(codec->sample_fmt) << 3;
         if (!bit_depth)
             bit_depth = codec->bits_per_coded_sample;