avcodec/dfa: Check dimension against maximum

The headers from where the dimensions are read in actual files
are limited to 16bit per component.

Fixes: Timeout
Fixes: 6305/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_DFA_fuzzer-4824270749302784

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

libavcodec/dfa.c

@@ -41,7 +41,7 @@ static av_cold int dfa_decode_init(AVCodecContext *avctx)
 
     avctx->pix_fmt = AV_PIX_FMT_PAL8;
 
-    if (!avctx->width || !avctx->height)
+    if (!avctx->width || !avctx->height || FFMAX(avctx->width, avctx->height) >= (1<<16))
         return AVERROR_INVALIDDATA;
 
     av_assert0(av_image_check_size(avctx->width, avctx->height, 0, avctx) >= 0);