Avoid timeout in avif_decode_fuzzer (#1972)

tests/oss-fuzz/avif_decode_fuzzer.cc

diff --git a/tests/oss-fuzz/avif_decode_fuzzer.cc b/tests/oss-fuzz/avif_decode_fuzzer.cc
index f4bd83a..a619465 100644
--- a/tests/oss-fuzz/avif_decode_fuzzer.cc
+++ b/tests/oss-fuzz/avif_decode_fuzzer.cc
@@ -15,7 +15,7 @@
   // timeouts and to speed the fuzzing up.
   // avifDecoderParse returns AVIF_RESULT_NOT_IMPLEMENTED if kImageSizeLimit is
   // bigger than AVIF_DEFAULT_IMAGE_SIZE_LIMIT.
-  constexpr uint32_t kImageSizeLimit = 8 * 1024 * 8 * 1024;
+  constexpr uint32_t kImageSizeLimit = 4 * 1024 * 4 * 1024;
   static_assert(kImageSizeLimit <= AVIF_DEFAULT_IMAGE_SIZE_LIMIT,
                 "Too big an image size limit");
   decoder->imageSizeLimit = kImageSizeLimit;