[mutter] egl: Allow passing NULL as error

[Jonas Ådahl <jadahl src gnome org>]

commit 18b9cf8a85cd967d260c0dc4cdc52552be20eae7
Author: Jonas Ådahl <jadahl gmail com>
Date:   Thu Oct 20 13:38:33 2016 +0800

    egl: Allow passing NULL as error
    
    When calling an EGL function is expected to sometimes fail, we might
    not care about the error.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=773629

 src/backends/meta-egl.c |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)
---
diff --git a/src/backends/meta-egl.c b/src/backends/meta-egl.c
index a1da5d2..5692012 100644
--- a/src/backends/meta-egl.c
+++ b/src/backends/meta-egl.c
@@ -127,6 +127,9 @@ set_egl_error (GError **error)
 {
   const char *error_str;
 
+  if (!error)
+    return;
+
   error_str = get_egl_error_str ();
   g_set_error (error, G_IO_ERROR,
                G_IO_ERROR_FAILED,