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

@@ -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);