avcodec/exr: Fix undefined integer multiplication

Fixes: signed integer overflow: 7020950083487072256 * 2 cannot be represented in type 'long long'
Fixes: 37523/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_EXR_fuzzer-5133634955771904

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

@@ -1033,12 +1033,14 @@ static int dwa_uncompress(EXRContext *s, const uint8_t *src, int compressed_size
     }
 
     if (ac_size > 0) {
-        unsigned long dest_len = ac_count * 2LL;
+        unsigned long dest_len;
         GetByteContext agb = gb;
 
         if (ac_count > 3LL * td->xsize * s->scan_lines_per_block)
             return AVERROR_INVALIDDATA;
 
+        dest_len = ac_count * 2LL;
+
         av_fast_padded_malloc(&td->ac_data, &td->ac_size, dest_len);
         if (!td->ac_data)
             return AVERROR(ENOMEM);
@@ -1062,12 +1064,14 @@ static int dwa_uncompress(EXRContext *s, const uint8_t *src, int compressed_size
     }
 
     {
-        unsigned long dest_len = dc_count * 2LL;
+        unsigned long dest_len;
         GetByteContext agb = gb;
 
         if (dc_count != dc_w * dc_h * 3)
             return AVERROR_INVALIDDATA;
 
+        dest_len = dc_count * 2LL;
+
         av_fast_padded_malloc(&td->dc_data, &td->dc_size, FFALIGN(dest_len, 64) * 2);
         if (!td->dc_data)
             return AVERROR(ENOMEM);