GML: fix bug that prevented multiple instanciation of the reader with Xerces backend (#5571)

git-svn-id: https://svn.osgeo.org/gdal/trunk@27530 f0d54148-0727-0410-94bb-9a71ac55c965
Test multiple instanciation of parser (#5571)

git-svn-id: https://svn.osgeo.org/gdal/trunk@27532 f0d54148-0727-0410-94bb-9a71ac55c965