Make sure having a comment before a <label/> doesn't break

Fixes:

runtime error: file
/home/karen/rhaptos.cnxmlutils/rhaptos/cnxmlutils/xsl/cnxml-to-html5.xsl
line 144 element attribute
xsl:attribute: Cannot add attributes to an element if children have been
already added to the element.

When we transform http://cnx.org/content/m46882/latest/index.cnxml