atrac3: Error on impossible encoding/channel combinations

Joint stereo encoded mono is impossible.

Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org

libavcodec/atrac3.c

@@ -941,9 +941,11 @@ static av_cold int atrac3_decode_init(AVCodecContext *avctx)
 
     if (q->coding_mode == STEREO)
         av_log(avctx, AV_LOG_DEBUG, "Normal stereo detected.\n");
-    else if (q->coding_mode == JOINT_STEREO)
+    else if (q->coding_mode == JOINT_STEREO) {
+        if (avctx->channels != 2)
+            return AVERROR_INVALIDDATA;
         av_log(avctx, AV_LOG_DEBUG, "Joint stereo detected.\n");
-    else {
+    } else {
         av_log(avctx, AV_LOG_ERROR, "Unknown channel coding mode %x!\n",
                q->coding_mode);
         return AVERROR_INVALIDDATA;