avcodec/cdgraphics: Use ff_set_dimensions()

Fixes: Timeout (17 sec -> 65 milli sec)
Fixes: 13264/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_CDGRAPHICS_fuzzer-5711167941509120

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

libavcodec/cdgraphics.c

@@ -81,11 +81,8 @@ static av_cold int cdg_decode_init(AVCodecContext *avctx)
         return AVERROR(ENOMEM);
     cc->transparency = -1;
 
-    avctx->width   = CDG_FULL_WIDTH;
-    avctx->height  = CDG_FULL_HEIGHT;
     avctx->pix_fmt = AV_PIX_FMT_PAL8;
-
-    return 0;
+    return ff_set_dimensions(avctx, CDG_FULL_WIDTH, CDG_FULL_HEIGHT);
 }
 
 static void cdg_border_preset(CDGraphicsContext *cc, uint8_t *data)