RFR: JDK-8077377 - java/net/MulticastSocket/SetOutgoingIf.java fails intermittently with NullPointerException

[Mark Sheppard]

Hi
    please oblige and review the following change
http://cr.openjdk.java.net/~msheppar/8077377/webrev/

to address the issue
https://bugs.openjdk.java.net/browse/JDK-8077377

it is possible that "stray" packets can be received in this 
MulticastSocket test, and
this results in a NPE from the NetworkInterface lookup based on the 
received address.
Change adds a test that the "from" NetworkInterface is not null.

regards
Mark