RFR: JDK-8066130 - com.sun.net.httpserver stop() throws NullPointerException if it is not started

[Mark Sheppard]

Hi,
    Please oblige and review the minor change
http://cr.openjdk.java.net/~msheppar/8066130/webrev/

to address the issue

https://bugs.openjdk.java.net/browse/JDK-8066130

a previous amendment to com.sun.net.httpserver.ServerImpl ( 
https://bugs.openjdk.java.net/browse/JDK-8015692)
failed to consider that a stop method may be invoked prior to a start 
method, hence assumed dispatcherThread would be set,
null check on disptacherThread added to stop 
com.sun.net.httpserver.ServerImpl stop method

regards
Mark