RFR 6594296: NetworkInterface.getHardwareAddress returns zero length byte array

[Chris Hegarty]

This is a conformance issue where getHardwareAddress may incorrectly 
return a zero length byte array, rather than null. The spec allows for 
null, in fact it requires it for such cases. The issue is reproducible 
with newer versions of Windows, with IPv6 enabled.

The problem is in src/windows/native/java/net/NetworkInterface_winXP.c. 
PhysicalAddressLength can be 0, in which case the byte array should not 
be created, rather than creating a 0 length byte[].

 From msdn:
   "PhysicalAddressLength

    Type: DWORD

    The length, in bytes, of the address specified in the
    PhysicalAddress member. For interfaces that do not have
    a data-link layer, this value is zero."

Webrev:
   http://cr.openjdk.java.net/~chegar/6594296/webrev.00/webrev/

-Chris.