RFR 7156085: ArrayIndexOutOfBoundsException throws in UTF8Reader of SAXParser

[Martin Buchholz]

Hi Joe,

I'd like you to do a code review.

http://cr.openjdk.java.net/~martin/webrevs/openjdk9/xerces-UTF8Reader-supplementary-characters/
https://bugs.openjdk.java.net/browse/JDK-7156085#comment-13569882
https://issues.apache.org/jira/browse/XERCESJ-1257

As usual, I don't know what I'm doing in jaxp land.
This bug should be fixed in both upstream xerces and in openjdk's copy.
Probably the xerces private UTF8 decoder should be thrown out, but I'm
only trying to bandaid the code here.