AXIOM-443: Ensure that OMSourcedElementImpl doesn't continue to use the parser returned by OMDataSource#getReader() after it has been closed. This causes no problem with Woodstox, but XLXP 1 correctly complains about this.

This fixes a regression caused by r1424378.


git-svn-id: https://svn.apache.org/repos/asf/webservices/commons/trunk/modules/axiom@1429539 13f79535-47bb-0310-9956-ffa450edef68