xmptag: Don't leak the namespace string if there are multiple

https://bugzilla.gnome.org/show_bug.cgi?id=775887

gst-libs/gst/tag/gstxmptag.c

@@ -1313,6 +1313,7 @@ gst_tag_list_from_xmp_buffer (GstBuffer * buffer)
                 }
                 if (ns_match[i].ns_prefix) {
                   if (strcmp (ns_map[i].original_ns, &as[6])) {
+                    g_free (ns_map[i].gstreamer_ns);
                     ns_map[i].gstreamer_ns = g_strdup (&as[6]);
                   }
                 }