RFR: 8079841: Buffer underflow with empty zip entry names

[Jeremy Manson]

There's a fairly harmless buffer underflow with empty zip names in libzip.
Martin has offered to sponsor this (right, Martin?).

We detected this with ASan, an automatic memory error and leak checking
tool built into Clang/LLVM.  This is one of the reasons that Alexander
Smundak asked if we could backport clang support to JDK8.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8079841

Webrev:
http://cr.openjdk.java.net/~jmanson/8079841/webrev.00/