[libxml2] Fix null pointer deref in docs with no root element

[Daniel Veillard <veillard src gnome org>]

commit b8e0fa3489b344d9cbd1165687e7dc5b15cc3727
Author: Hugh Davenport <hugh davenport net nz>
Date:   Wed May 4 10:55:49 2016 +0800

        Fix null pointer deref in docs with no root element
    
        From https://bugzilla.gnome.org/show_bug.cgi?id=758514

 xmllint.c |    6 ++++++
 1 files changed, 6 insertions(+), 0 deletions(-)
---
diff --git a/xmllint.c b/xmllint.c
index 9e85c11..00f1769 100644
--- a/xmllint.c
+++ b/xmllint.c
@@ -2002,6 +2002,12 @@ static void walkDoc(xmlDocPtr doc) {
     xmlNsPtr ns;
 
     root = xmlDocGetRootElement(doc);
+    if (root == NULL ) {
+        xmlGenericError(xmlGenericErrorContext,
+                "Document does not have a root element");
+        progresult = XMLLINT_ERR_UNCLASS;
+        return;
+    }
     for (ns = root->nsDef, i = 0;ns != NULL && i < 20;ns=ns->next) {
         namespaces[i++] = ns->href;
         namespaces[i++] = ns->prefix;