Hardening of XPath evaluation

Add a mechanism of frame for XPath evaluation when entering a function
or a scoped evaluation, also fix a potential problem in predicate
evaluation.
diff --git a/xpointer.c b/xpointer.c
index 7a42d02..37afa3a 100644
--- a/xpointer.c
+++ b/xpointer.c
@@ -1269,6 +1269,7 @@
 	ctxt->valueNr = 0;
 	ctxt->valueMax = 10;
 	ctxt->value = NULL;
+	ctxt->valueFrame = 0;
     }
     SKIP_BLANKS;
     if (CUR == '/') {