avcodec/exr: Fix overflow with many blocks

Fixes: signed integer overflow: 1073741827 * 8 cannot be represented in type 'int'
Fixes: 25621/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_EXR_fuzzer-6304841641754624

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

@@ -1789,7 +1789,7 @@ static int decode_frame(AVCodecContext *avctx, void *data,
     if ((ret = ff_thread_get_buffer(avctx, &frame, 0)) < 0)
         return ret;
 
-    if (bytestream2_get_bytes_left(&s->gb) < nb_blocks * 8)
+    if (bytestream2_get_bytes_left(&s->gb)/8 < nb_blocks)
         return AVERROR_INVALIDDATA;
 
     // check offset table and recreate it if need