avcodec/exr: check ac_size

Fixes: out of array read
Fixes: dwa_uncompress.py.crash.exr

The code will read from the ac data even if ac_size is 0, thus that case
is not implemented and we ask for a sample and error out cleanly

Found-by: Google Big Sleep

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 8e078826da6f2a1dffa25162121b43b272f5e5fa)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/exr.c

@@ -1015,6 +1015,11 @@ static int dwa_uncompress(const EXRContext *s, const uint8_t *src, int compresse
     )
         return AVERROR_INVALIDDATA;
 
+    if (ac_size <= 0) {
+        avpriv_request_sample(s->avctx, "Zero ac_size");
+        return AVERROR_INVALIDDATA;
+    }
+
     if ((uint64_t)rle_raw_size > INT_MAX) {
         avpriv_request_sample(s->avctx, "Too big rle_raw_size");
         return AVERROR_INVALIDDATA;