RFR: Inet[4|6]Address native initializing code should check fieldID values

[Chris Hegarty]

Another installment of checks for return values from
GetFieldID, and friends, to follow up on last weeks work [1].

http://cr.openjdk.java.net/~chegar/netNullChecks/webrev/

There are more cleanups to come along the same lines, but I'd like to 
keep these changes small and confined for now.

-Chris.

[1] http://mail.openjdk.java.net/pipermail/net-dev/2013-October/007698.html