RFR JDK-8146431:j.u.z.ZipFile.getEntry("") throws AIOOBE

[Xueming Shen]

Hi, please help review the change for JDK-8146431

Issue: https://bugs.openjdk.java.net/browse/JDK-8146431
webrev: http://cr.openjdk.java.net/~sherman/8146431

Cause:

The parameter "ulen" in native version zip_util.c.ZIP_GetEntry() serves two
purposes, (1) the length of the name (2) whether or not to try adding a
slash if first try failed. It has an explicit check as

        /* If no real length was passed in, we are done */
        if (ulen == 0) {
            break;
        }

The new java implementation uses and checks the flag "addSlash", but it
still needs to check the "name.len" before adding any slash for next try.

Thanks,
Sherman