change suggested by Anthony Carrico when unregistering a namespace prefix

* xpath.c: change suggested by Anthony Carrico when unregistering
  a namespace prefix to a context
* hash.c: be more careful about calling callbacks with NULL payloads.
Daniel
diff --git a/xpath.c b/xpath.c
index 6e7e471..627fa20 100644
--- a/xpath.c
+++ b/xpath.c
@@ -2974,6 +2974,9 @@
 	ctxt->nsHash = xmlHashCreate(10);
     if (ctxt->nsHash == NULL)
 	return(-1);
+    if (ns_uri == NULL)
+        return(xmlHashRemoveEntry(ctxt->nsHash, ns_uri,
+	                          (xmlHashDeallocator)xmlFree));
     return(xmlHashUpdateEntry(ctxt->nsHash, prefix, (void *) xmlStrdup(ns_uri),
 			      (xmlHashDeallocator)xmlFree));
 }