[libxml2] Fix out-of-bounds read with 'xmllint --htmlout'

[Nick Wellnhofer <nwellnhof src gnome org>]

commit 50f06b3efb638efb0abd95dc62dca05ae67882c2
Author: Nick Wellnhofer <wellnhofer aevum de>
Date:   Fri Aug 7 21:54:27 2020 +0200

    Fix out-of-bounds read with 'xmllint --htmlout'
    
    Make sure that truncated UTF-8 sequences don't cause an out-of-bounds
    array access.
    
    Thanks to @SuhwanSong and the Agency for Defense Development (ADD) for
    the report.
    
    Fixes #178.

 xmllint.c | 6 ++++++
 1 file changed, 6 insertions(+)
---
diff --git a/xmllint.c b/xmllint.c
index f6a8e4636..c647486f3 100644
--- a/xmllint.c
+++ b/xmllint.c
@@ -528,6 +528,12 @@ static void
 xmlHTMLEncodeSend(void) {
     char *result;
 
+    /*
+     * xmlEncodeEntitiesReentrant assumes valid UTF-8, but the buffer might
+     * end with a truncated UTF-8 sequence. This is a hack to at least avoid
+     * an out-of-bounds read.
+     */
+    memset(&buffer[sizeof(buffer)-4], 0, 4);
     result = (char *) xmlEncodeEntitiesReentrant(NULL, BAD_CAST buffer);
     if (result) {
        xmlGenericError(xmlGenericErrorContext, "%s", result);