avcodec/arbc: Skip unchanged frames

Fixes: Timeout (16sec -> 5sec)
Fixes: 14128/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_ARBC_fuzzer-5767365721063424

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

libavcodec/arbc.c

@@ -121,7 +121,7 @@ static int decode_frame(AVCodecContext *avctx, void *data,
     bytestream2_skip(&s->gb, 8);
     nb_segments = bytestream2_get_le16(&s->gb);
     if (nb_segments == 0)
-        keyframe = 0;
+        return avpkt->size;
 
     if (7 * nb_segments > bytestream2_get_bytes_left(&s->gb))
         return AVERROR_INVALIDDATA;