fixed a serious memory problen when walking the namespace axis showing up

* xpath.c include/libxml/xpath.h: fixed a serious memory problen
  when walking the namespace axis showing up in
  libxst/tests/general/bug-12
* xmlmemory.c: added the possibility to trace a given block
  defined by its address
Daniel
diff --git a/include/libxml/xpath.h b/include/libxml/xpath.h
index 8d921f8..78cb0ca 100644
--- a/include/libxml/xpath.h
+++ b/include/libxml/xpath.h
@@ -235,6 +235,10 @@
     /* function lookup function and data */
     void *funcLookupFunc;		/* function lookup func */
     void *funcLookupData;		/* function lookup data */
+
+    /* temporary namespace lists kept for walking the namespace axis */
+    xmlNsPtr *tmpNsList;		/* Array of namespaces */
+    int tmpNsNr;			/* number of namespace in scope */
 };
 
 /*