Fix a potential freeing error in XPath

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;
 	    }