[libxslt] Fix memory leak in str:concat with empty node-set

[Nick Wellnhofer <nwellnhof src gnome org>]

commit b89ec9354171aeef76ceba67ebc4012703a88370
Author: Nick Wellnhofer <wellnhofer aevum de>
Date:   Sat May 20 22:36:50 2017 +0200

    Fix memory leak in str:concat with empty node-set
    
    Found with libFuzzer and ASan.

 libexslt/strings.c |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/libexslt/strings.c b/libexslt/strings.c
index d55ec44..2488936 100644
--- a/libexslt/strings.c
+++ b/libexslt/strings.c
@@ -522,6 +522,7 @@ exsltStrConcatFunction (xmlXPathParserContextPtr ctxt, int nargs) {
     obj = valuePop (ctxt);
 
     if (xmlXPathNodeSetIsEmpty(obj->nodesetval)) {
+        xmlXPathFreeObject(obj);
        xmlXPathReturnEmptyString(ctxt);
        return;
     }