avcodec/exr: Check ac_count

Fixes: signed integer overflow: -9223372036854775808 * 2 cannot be represented in type 'long long'
Fixes: 36244/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_EXR_fuzzer-6090656186499072

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

libavcodec/exr.c

@@ -1014,7 +1014,9 @@ static int dwa_uncompress(EXRContext *s, const uint8_t *src, int compressed_size
     dc_count = AV_RL64(src + 72);
     ac_compression = AV_RL64(src + 80);
 
-    if (compressed_size < (uint64_t)(lo_size | ac_size | dc_size | rle_csize) || compressed_size < 88LL + lo_size + ac_size + dc_size + rle_csize)
+    if (   compressed_size < (uint64_t)(lo_size | ac_size | dc_size | rle_csize) || compressed_size < 88LL + lo_size + ac_size + dc_size + rle_csize
+        || ac_count > (uint64_t)INT_MAX/2
+    )
         return AVERROR_INVALIDDATA;
 
     bytestream2_init(&gb, src + 88, compressed_size - 88);