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Code Review

This pull request modifies the APNG image decoding logic to handle fdAT chunks with short data lengths. In APNGImageGenerator::DemuxNextImage, a check is added to ensure the data_length of an fdAT chunk is at least 4 bytes. If the length is less than 4, the function now returns an error, preventing a potential integer underflow and subsequent buffer overflow in memcpy. A new test file, image_generator_apng_unittests.cc, is added with two tests: one that verifies that an APNG with an fdAT chunk of length 0 is rejected, and another that confirms an APNG with an fdAT chunk of length 4 is accepted. The BUILD.gn file is updated to include the new test file and its associated image fixtures.