RFR: JDK-8239139 testmake fail with warning about strncpy using gcc version 8

[linzang(臧琳)]

Hi,
     May I ask help to review one tiny fix of build failure described at https://bugs.openjdk.java.net/browse/JDK-8239139
     Root cause is gcc version 8 prints warning for strncpy.
     The fix simply replace strncpy with snprintf.
     Thanks!
     Bug: https://bugs.openjdk.java.net/browse/JDK-8239139
     webrev: http://cr.openjdk.java.net/~lzang/8239139/webrev/

BRs,
Lin