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/APIchunk20.html b/doc/APIchunk20.html
index cd7a113..d7718bb 100644
--- a/doc/APIchunk20.html
+++ b/doc/APIchunk20.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 p:</h2><dl><dt>pairs</dt><dd><a href="html/libxml-SAX.html#startElement">startElement</a><br />
 <a href="html/libxml-parser.html#startElementNsSAX2Func">startElementNsSAX2Func</a><br />
 <a href="html/libxml-parser.html#startElementSAXFunc">startElementSAXFunc</a><br />
@@ -495,4 +496,5 @@
 <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>