If an attribute-set reference (via "use-attribute-set") cannot
be resolved, then in xsltMergeAttrElemList(), the strings
in the fields @set and @ns fields of xsltAttrElem are duplicated,
but never freed.
On the other hand, if the reference can be resolved, then those
duplicated strings are freed in xsltResolveSASCallback().

I think the best way to fix this is not to duplicate the
strings, but to use the string-dict.

Example:
=======
<?xml version='1.0'?>
<xsl:stylesheet version="1.0"
    xmlns:xsl="http://www.w3.org/1999/XSL/Transform">

  <xsl:attribute-set name="attr-set-2" use-attribute-sets="attr-set-1">
  </xsl:attribute-set>  

</xsl:stylesheet>

xsltproc results
================
$xsltproc attr-set-2.xsl
xsl:attribute-set : use-attribute-sets attr-set-2 reference missing attr-set-1

memory dump
===========
$cat .memdump
      MEMORY ALLOCATED : 11, MAX was 20817
BLOCK  NUMBER   SIZE  TYPE
0         335     11 malloc()  in none(0) "attr-set-1"