[libxml2] Fix potential memory leak in xmlBufBackToBuffer

[Nick Wellnhofer <nwellnhof src gnome org>]

commit 5f1f455c2f659eb6c82adf2b1d3ae00466367873
Author: Nick Wellnhofer <wellnhofer aevum de>
Date:   Fri Sep 13 15:51:16 2019 +0200

    Fix potential memory leak in xmlBufBackToBuffer
    
    Fixes bug #794373
    https://bugzilla.gnome.org/show_bug.cgi?id=794373
    
    Also see merge request !42

 buf.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/buf.c b/buf.c
old mode 100644
new mode 100755
index 21cb9d80..6a8721bc
--- a/buf.c
+++ b/buf.c
@@ -1204,10 +1204,10 @@ xmlBufferPtr
 xmlBufBackToBuffer(xmlBufPtr buf) {
     xmlBufferPtr ret;
 
-    if ((buf == NULL) || (buf->error))
+    if (buf == NULL)
         return(NULL);
     CHECK_COMPAT(buf)
-    if (buf->buffer == NULL) {
+    if ((buf->error) || (buf->buffer == NULL)) {
         xmlBufFree(buf);
         return(NULL);
     }