RFR 8026880: NetworkInterface native initializing code should check fieldID values

[Chris Hegarty]

This is a trivial change, good practice, to check return values from 
GetFieldID, and friends.

I've taken a quick look over the networking native code and see no other 
similar issues.

http://cr.openjdk.java.net/~chegar/8026880/webrev.00/

-Chris.