High: xml.c: fix cleaning of own vs. XSLT's XML node's _private load

The issue is described in the in-code comment. Previously, this wasn't
a problem as no result tree fragments got created with upgrade-1.3.xsl.
With upgrade-2.10.xsl, they are used heavily, mostly as oracles whether
the evaluated content will be non-empty and (or in addition) to avoid
redundant evaluations.