zip_archive.cc: Use static cast instead of masking

Although ubsan's implicit-unsigned-integer-truncation sanitizer may be
happy, this code still performs an implicit conversion from a wider
width data structure to a narrower width data structure. Rather than
masking the bits, make the conversion explicit. This keeps ubsan happy
as well as addressing a -Wconversion warning.

This change addresses comments from the post-submit review
of a4e54336.

Test: compiles and boots.
Bug: 122975762
Change-Id: I1fa6d6f8a6fcfb93ba9916b7d2b3564ca1d8caf3