avcodec/mpegaudiodec_template: Correct return code on id3 tag discarding

Fixes: 665/clusterfuzz-testcase-4863789881098240

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/targets/ffmpegSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/mpegaudiodec_template.c

@@ -1665,7 +1665,7 @@ static int decode_frame(AVCodecContext * avctx, void *data, int *got_frame_ptr,
     header = AV_RB32(buf);
     if (header>>8 == AV_RB32("TAG")>>8) {
         av_log(avctx, AV_LOG_DEBUG, "discarding ID3 tag\n");
-        return buf_size;
+        return buf_size + skipped;
     }
     ret = avpriv_mpegaudio_decode_header((MPADecodeHeader *)s, header);
     if (ret < 0) {