[libxml2] Fix commit "Memory leak in xmlFreeID (xmlreader.c)"

[Nick Wellnhofer <nwellnhof src gnome org>]

commit 26828cb3a1294e09e42064f4769d4b3c0a8623b3
Author: Nick Wellnhofer <wellnhofer aevum de>
Date:   Mon Jan 7 16:52:42 2019 +0100

    Fix commit "Memory leak in xmlFreeID (xmlreader.c)"
    
    The recent commit "Memory leak in xmlFreeID (xmlreader.c)" introduced
    a double-free.

 xmlreader.c | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/xmlreader.c b/xmlreader.c
index 3acec759..cd1fb5fe 100644
--- a/xmlreader.c
+++ b/xmlreader.c
@@ -273,6 +273,7 @@ xmlTextReaderRemoveID(xmlDocPtr doc, xmlAttrPtr attr) {
        return(-1);
     }
     id->name = attr->name;
+    attr->name = NULL;
     id->attr = NULL;
     return(0);
 }