[libxml2] Fix potential out of bound access

[Daniel Veillard <veillard src gnome org>]

commit 6a36fbe3b3e001a8a840b5c1fdd81cefc9947f0d
Author: Daniel Veillard <veillard redhat com>
Date:   Mon Oct 29 10:39:55 2012 +0800

    Fix potential out of bound access

 parser.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/parser.c b/parser.c
index 0d8d7f2..bd634e9 100644
--- a/parser.c
+++ b/parser.c
@@ -4076,7 +4076,7 @@ xmlParseAttValueComplex(xmlParserCtxtPtr ctxt, int *attlen, int normalize) {
         goto error;
 
     if ((in_space) && (normalize)) {
-        while (buf[len - 1] == 0x20) len--;
+        while ((len > 0) && (buf[len - 1] == 0x20)) len--;
     }
     buf[len] = 0;
     if (RAW == '<') {