RFR [14/java.xml] 8230094: CCE in createXMLEventWriter(Result) over an arbitrary XMLStreamWriter

[Joe Wang]

Please review a patch for an issue where a custom impl of 
XMLStreamWriter caused a CCE. The problem was that our implementation 
was changed to an internally extended interface. We could consider 
making it external, but for now, we'll do with a check.

Note that there's no material change in XMLOutputFactoryImpl.java except 
a minor one at line 160.

JBS: https://bugs.openjdk.java.net/browse/JDK-8230094
webrev: http://cr.openjdk.java.net/~joehw/jdk14/8230094/webrev/

Thanks,
Joe