Avoid string decoding in ZipFile.Source.getEntryPos

[Eirik Bjørsnøs]

Hi,

Please review this PR which speeds up ZipFile entry lookup by ~17%, mostly
by removing a string decode / allocation when getEntryPos compares lookup
names with names in the CEN:

https://github.com/openjdk/jdk/pull/12290

Thanks,
Eirik.