[cogl] cogl-buffer: Stop warning spam running in sw rendering

[Robert Bragg <rbragg src gnome org>]

commit 83dbf79986981fac9ec0f2575b7c7cb32f629f0f
Author: Bastien Nocera <hadess hadess net>
Date:   Mon Feb 25 11:26:49 2013 +0100

    cogl-buffer: Stop warning spam running in sw rendering
    
    /etc/gdm/Xsession[731]: Aviso do gerenciador de janelas: Log level 8:
    g_error_free: assertion `error != NULL' failed
    
    https://bugzilla.gnome.org/show_bug.cgi?id=694657
    
    Reviewed-by: Robert Bragg <robert linux intel com>

 cogl/cogl-buffer.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)
---
diff --git a/cogl/cogl-buffer.c b/cogl/cogl-buffer.c
index 4edfa54..6aa10dd 100644
--- a/cogl/cogl-buffer.c
+++ b/cogl/cogl-buffer.c
@@ -290,7 +290,8 @@ _cogl_buffer_map_range_for_fill_or_fallback (CoglBuffer *buffer,
   if (ret)
     return ret;
 
-  cogl_error_free (ignore_error);
+  if (ignore_error)
+    cogl_error_free (ignore_error);
 
   /* If the map fails then we'll use a temporary buffer to fill
      the data and then upload it using cogl_buffer_set_data when