RFR JDK-8244961: MethodHandles::privateLookupIn throws NPE when called during initPhase2

[Mandy Chung]

MethodHandles::privateLookupIn should prepare for being called during 
early startup when the module of java.base classes are not yet 
assigned.  This bug is uncovered by panama prototype converting NIO to 
use memory access API.

The fix is very simple and move the call to Module::isNamed only if the 
caller module and target module are different:
http://cr.openjdk.java.net/~mchung/jdk15/webrevs/8244961/webrev.00/

Mandy