RFR 8157811 [9] Additional minor fixes and cleanups in NetworkInterface.c

[Chris Hegarty]

As a follow up to JDK-8156521, and when comparing against the
version of this file in jdk8u-dev a few minor issues were noticed. 

There is a free of a memory structure that was missed somehow
in the 9 version of this file ( it is already in the 8 version ). The
remaining few changes are just some minor cleanup.

http://cr.openjdk.java.net/~chegar/8157811/
https://bugs.openjdk.java.net/browse/JDK-8157811

-Chris.