fix ZipCentralDir warning

[xuwei-k]

https://bugs.openjdk.org/browse/JDK-8299995

[warn] /home/runner/work/zinc/zinc/internal/zinc-classfile/src/main/java/sbt/internal/inc/zip/ZipCentralDir.java:86:28: possible 'this' escape before subclass is fully initialized
[warn] readEntries()
[warn]              ^
[warn] /home/runner/work/zinc/zinc/internal/zinc-classfile/src/main/java/sbt/internal/inc/zip/ZipCentralDir.java:122:36: previous possible 'this' escape happens here via invocation
[warn] Entry.readCEN(this, inode.pos)
[warn]                               ^
[warn] /home/runner/work/zinc/zinc/internal/zinc-classfile/src/main/java/sbt/internal/inc/zip/ZipCentralDir.java:486:35: previous possible 'this' escape happens here via invocation
[warn] new Entry().cen(zipfs, pos)
[warn]                            ^
[warn] /home/runner/work/zinc/zinc/internal/zinc-classfile/src/main/java/sbt/internal/inc/zip/ZipCentralDir.java:518:27: previous possible 'this' escape happens here via invocation
[warn] zipfs
[warn]      ^