[libxslt] Fix NULL pointer deref in xsltFormatNumberFunction

[Nick Wellnhofer <nwellnhof src gnome org>]

commit 8d1f7147c34c989e431826505854cd328244293b
Author: Nick Wellnhofer <wellnhofer aevum de>
Date:   Wed May 17 13:10:05 2017 +0200

    Fix NULL pointer deref in xsltFormatNumberFunction
    
    Introduced recently with my commit
    
        38d4a90 Make xsl:decimal-format work with namespaces
    
    Coverity CID 1434477. Also found with libFuzzer and ASan.

 libxslt/functions.c |    4 +++-
 1 files changed, 3 insertions(+), 1 deletions(-)
---
diff --git a/libxslt/functions.c b/libxslt/functions.c
index 5cc9da5..43d82f6 100644
--- a/libxslt/functions.c
+++ b/libxslt/functions.c
@@ -632,7 +632,9 @@ xsltFormatNumberFunction(xmlXPathParserContextPtr ctxt, int nargs)
                 sheet->errors++;
                 ncname = NULL;
             }
-            nsUri = ns->href;
+            else {
+                nsUri = ns->href;
+            }
         }
         if (ncname != NULL) {
            formatValues = xsltDecimalFormatGetByQName(sheet, nsUri, ncname);