RFR (XS): 8231600: Avoid GCC 8 strncpy error in jdk.jdwp.agent

[Florian Weimer]

Bug: https://bugs.openjdk.java.net/browse/JDK-8231600
Webrev: http://cr.openjdk.java.net/~fweimer/8231600/webrev.01/

We know the size of the string, so we can use memcpy instead of strncpy.

Thanks,
Florian