Don't explode if NetworkInterface.getByInetAddress does.

Certain Motorola RAZRs always throw a SocketException when
you call that method. We don't _really_ need a perfect MTU,
so for those devices just return the default MTU.
Merge pull request #188 from square/jwilson/fix_razr_bug

Don't explode if NetworkInterface.getByInetAddress does.