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/globals.c b/globals.c
index 69b4dc2..f1aa150 100644
--- a/globals.c
+++ b/globals.c
@@ -39,11 +39,21 @@
  */
 static xmlMutexPtr xmlThrDefMutex = NULL;
 
+/**
+ * xmlInitGlobals:
+ *
+ * Additional initialisation for multi-threading
+ */
 void xmlInitGlobals()
 {
     xmlThrDefMutex = xmlNewMutex();
 }
 
+/**
+ * xmlCleanupGlobals:
+ *
+ * Additional cleanup for multi-threading
+ */
 void xmlCleanupGlobals()
 {
     if (xmlThrDefMutex != NULL) {