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/apibuild.py b/doc/apibuild.py
index b2543c0..94720f3 100755
--- a/doc/apibuild.py
+++ b/doc/apibuild.py
@@ -1793,6 +1793,8 @@
 	if letter != None:
 	    output.write("      </letter>\n")
 	    output.write("    </chunk>\n")
+	    if count != 0:
+	        chunks.append(["chunk%s" % (chunk -1), first_letter, letter])
 	    output.write("    <chunks>\n")
 	    for ch in chunks:
 		output.write("      <chunk name='%s' start='%s' end='%s'/>\n" % (