avcodec/cinepak: Skip empty frames

Speeds up decoding from 3 to 0.1 seconds for 6302/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_CINEPAK_fuzzer-5626371985375232
Fixes: Timeout

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

libavcodec/cinepak.c

@@ -444,6 +444,7 @@ static int cinepak_decode_frame(AVCodecContext *avctx,
     const uint8_t *buf = avpkt->data;
     int ret = 0, buf_size = avpkt->size;
     CinepakContext *s = avctx->priv_data;
+    int num_strips;
 
     s->data = buf;
     s->size = buf_size;
@@ -451,6 +452,12 @@ static int cinepak_decode_frame(AVCodecContext *avctx,
     if (s->size < 10)
         return AVERROR_INVALIDDATA;
 
+    num_strips = AV_RB16 (&s->data[8]);
+
+    //Empty frame, do not waste time
+    if (!num_strips && (!s->palette_video || !av_packet_get_side_data(avpkt, AV_PKT_DATA_PALETTE, NULL)))
+        return buf_size;
+
     if ((ret = cinepak_predecode_check(s)) < 0) {
         av_log(avctx, AV_LOG_ERROR, "cinepak_predecode_check failed\n");
         return ret;