avcodec/rl2: set dimensions

The dimensions are always 320x200 they are hardcoded in the demuxer.
Hardcode them instead in the decoder.

Fixes: Timeout (16sec -> 400ms)
Fixes: 15574/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_RL2_fuzzer-5158614072819712

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

libavcodec/rl2.c

@@ -134,10 +134,15 @@ static av_cold int rl2_decode_init(AVCodecContext *avctx)
     Rl2Context *s = avctx->priv_data;
     int back_size;
     int i;
+    int ret;
 
     s->avctx       = avctx;
     avctx->pix_fmt = AV_PIX_FMT_PAL8;
 
+    ret = ff_set_dimensions(avctx, 320, 200);
+    if (ret < 0)
+        return ret;
+
     /** parse extra data */
     if (!avctx->extradata || avctx->extradata_size < EXTRADATA1_SIZE) {
         av_log(avctx, AV_LOG_ERROR, "invalid extradata size\n");