libzipfile: add additional validity checks.

- Make sure the start and end of the data for a given entry are
  within the allocated buffer.
- Make sure all central directory entries start and end within the
  central directory buffer.
- Reject zip file entries that have no filenames.

bug: 30916186
test: test_zipfile with known bad zip files.
Change-Id: Ibf3f6469e60c85ec1608f5ce613d40867d2d09b7