RFR 8157797: SAX Parser throws incorrect error on invalid xml

[Joe Wang]

Hi,

Please review a quick fix for an incorrect error that should never have 
been thrown.

JBS: https://bugs.openjdk.java.net/browse/JDK-8157797

webrev: http://cr.openjdk.java.net/~joehw/jdk9/8157797/webrev/

Thanks,
Joe