MXParser do not fail when encountering invalid characters in comments

[belingueres]

Discovered with the help of https://github.com/belingueres/xml-conformance

MXParser fails on parts of the W3C XML Conformance Test Suite.

In this issue, those particular cases from "IBM XML Conformance Test Suite - Production 2" (see https://github.com/belingueres/xml-conformance/blob/main/src/test/resources/xmlconf/ibm/ibm_oasis_not-wf.xml) regarding the inclusion of invalid characters inside comments.