zip_archive: Fix a few more badly written tests.

Fix failures in DataDescriptor related tests due to to a bad call to
SetZipString (undefined behaviour). Also fix a typo in the test for
invalid descriptors, we were asserting things on the wrong array.

Test: zip_archive_test
Change-Id: I8c9a632443fdf1d5c115670d6e9317e1f4bf6ef4