[libxml2] Fix a potential freeing error in XPath

[Daniel Veillard <veillard src gnome org>]

commit df83c17e5a2646bd923f75e5e507bc80d73c9722
Author: Daniel Veillard <veillard redhat com>
Date:   Wed Nov 17 14:12:14 2010 +0100

    Fix a potential freeing error in XPath

 xpath.c |   12 ++++++++----
 1 files changed, 8 insertions(+), 4 deletions(-)
---
diff --git a/xpath.c b/xpath.c
index 81e33f6..1447be5 100644
--- a/xpath.c
+++ b/xpath.c
@@ -11763,11 +11763,15 @@ xmlXPathCompOpEvalPositionalPredicate(xmlXPathParserContextPtr ctxt,
 
 	    if ((ctxt->error != XPATH_EXPRESSION_OK) || (res == -1)) {
 	        xmlXPathObjectPtr tmp;
-		/* pop the result */
+		/* pop the result if any */
 		tmp = valuePop(ctxt);
-		xmlXPathReleaseObject(xpctxt, tmp);
-		/* then pop off contextObj, which will be freed later */
-		valuePop(ctxt);
+                if (tmp != contextObj)
+                    /*
+                     * Free up the result
+                     * then pop off contextObj, which will be freed later
+                     */
+                    xmlXPathReleaseObject(xpctxt, tmp);
+                    valuePop(ctxt);
 		goto evaluation_error;
 	    }