zip_writer_test.cc: fix implicit integer truncation

converting from a size_t to a uint8_t results in a loss of precision,
which triggers ubsan's implicit-unsigned-integer-truncation checker.
Make the conversion explicit.

This change allows the ziparchive-tests to pass with ubsan enabled.

Test: atest ziparchive-tests
Test: compiles and boots
Bug: 122975762
Change-Id: I63f28b58f1ca1f4c57323494cb1f4a41e0f34fba