fixed xmlXPathReturnString to cater for NULL pointer (bug 129561) added

* include/libxml/xpathInternals.h: fixed xmlXPathReturnString
  to cater for NULL pointer (bug 129561)
* globals.c: added comment to suppress documentation warning
* doc/apibuild.py: fixed problem which caused last APIchunkxx.html
  to be lost.  Rebuilt doc/* (including adding APIchunk26.html)
diff --git a/doc/APIchunk13.html b/doc/APIchunk13.html
index 698cc4c..5756973 100644
--- a/doc/APIchunk13.html
+++ b/doc/APIchunk13.html
@@ -33,6 +33,7 @@
 <a href="APIchunk23.html">t-t</a>
 <a href="APIchunk24.html">u-w</a>
 <a href="APIchunk25.html">x-x</a>
+<a href="APIchunk26.html">y-z</a>
 </h2><h2>Letter f:</h2><dl><dt>fTP</dt><dd><a href="html/libxml-nanoftp.html#xmlNanoFTPConnectTo">xmlNanoFTPConnectTo</a><br />
 </dd><dt>facet</dt><dd><a href="html/libxml-schemasInternals.html#XML_SCHEMAS_FACET_COLLAPSE">XML_SCHEMAS_FACET_COLLAPSE</a><br />
 <a href="html/libxml-schemasInternals.html#XML_SCHEMAS_FACET_PRESERVE">XML_SCHEMAS_FACET_PRESERVE</a><br />
@@ -386,4 +387,5 @@
 <a href="APIchunk23.html">t-t</a>
 <a href="APIchunk24.html">u-w</a>
 <a href="APIchunk25.html">x-x</a>
+<a href="APIchunk26.html">y-z</a>
 </h2><p><a href="bugs.html">Daniel Veillard</a></p></td></tr></table></td></tr></table></td></tr></table></td></tr></table></td></tr></table></body></html>