alac: do not return success if nothing was decoded

If we encounter an END element before anything is decoded, we would
return success even though the output frame has not been allocated,
which is invalid.

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

libavcodec/alac.c

@@ -447,6 +447,10 @@ static int alac_decode_frame(AVCodecContext *avctx, void *data,
         av_log(avctx, AV_LOG_ERROR, "no end tag found. incomplete packet.\n");
         return AVERROR_INVALIDDATA;
     }
+    if (!alac->nb_samples) {
+        av_log(avctx, AV_LOG_ERROR, "No decodable data in the packet\n");
+        return AVERROR_INVALIDDATA;
+    }
 
     if (avpkt->size * 8 - get_bits_count(&alac->gb) > 8) {
         av_log(avctx, AV_LOG_ERROR, "Error : %d bits left\n",