RFR: 8223457 java.net.ServerSocket protected constructor should throw NPE if impl null

[Michael McMahon]

Could I get the following small change reviewed please?
The recent change in the SocketImpl code unmasked the fact that
the protected constructor of ServerSocket was not checking for a null
impl parameter as required.

http://cr.openjdk.java.net/~michaelm/8223457/webrev.1/

Thanks
Michael