libopusdec: fix out-of-bounds read

Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>

libavcodec/libopusdec.c

@@ -48,6 +48,13 @@ static av_cold int libopus_decode_init(AVCodecContext *avc)
         avc->channels = 2;
     }
 
+    avc->channels = avc->extradata_size >= 10 ? avc->extradata[9] : (avc->channels == 1) ? 1 : 2;
+    if (avc->channels <= 0) {
+        av_log(avc, AV_LOG_WARNING,
+               "Invalid number of channels %d, defaulting to stereo\n", avc->channels);
+        avc->channels = 2;
+    }
+
     avc->sample_rate    = 48000;
     avc->sample_fmt     = avc->request_sample_fmt == AV_SAMPLE_FMT_FLT ?
                           AV_SAMPLE_FMT_FLT : AV_SAMPLE_FMT_S16;