graphics/libavif: unbreak tests on 32-bit archs after 36a733d8a533

tests/gtest/avifincrtest_helpers.cc:136:28: error: implicit conversion loses integer precision: 'unsigned long long' to 'size_t' (aka 'unsigned int') [-Werror,-Wshorten-64-to-32]

size = data->full_size - offset;
     ~ ~~~~~~~~~~~~~~~~^~~~~~~~