[mutter/gnome-3-34] x11: Do not trust there is task in error paths

[Robert Mader <rmader src gnome org>]

commit ebfe5e44c22b462d47a2c877a8a3455386357204
Author: Carlos Garnacho <carlosg gnome org>
Date:   Wed Apr 15 10:46:34 2020 +0200

    x11: Do not trust there is task in error paths
    
    Flushing the X11 selection output stream may happen synchronously or
    implicitly, in which case there is not a task to complete. Check there
    is actually a task before returning errors. We additionally set the
    pipe_error flag, so future operations will fail with an error, albeit
    with a more generic message.
    
    https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1198
    
    (cherry picked from commit 1909977a6787d6e06d4931df158b821f6fb35ad1)

 src/x11/meta-x11-selection-output-stream.c | 17 ++++++++++-------
 1 file changed, 10 insertions(+), 7 deletions(-)
---
diff --git a/src/x11/meta-x11-selection-output-stream.c b/src/x11/meta-x11-selection-output-stream.c
index aa2b78c47..4ccebd2ab 100644
--- a/src/x11/meta-x11-selection-output-stream.c
+++ b/src/x11/meta-x11-selection-output-stream.c
@@ -286,13 +286,16 @@ meta_x11_selection_output_stream_perform_flush (MetaX11SelectionOutputStream *st
       priv->delete_pending = FALSE;
       priv->pipe_error = TRUE;
 
-      XGetErrorText (xdisplay, error_code, error_str, sizeof (error_str));
-      g_task_return_new_error (priv->pending_task,
-                               G_IO_ERROR,
-                               G_IO_ERROR_BROKEN_PIPE,
-                               "Failed to flush selection output stream: %s",
-                               error_str);
-      g_clear_object (&priv->pending_task);
+      if (priv->pending_task)
+        {
+          XGetErrorText (xdisplay, error_code, error_str, sizeof (error_str));
+          g_task_return_new_error (priv->pending_task,
+                                   G_IO_ERROR,
+                                   G_IO_ERROR_BROKEN_PIPE,
+                                   "Failed to flush selection output stream: %s",
+                                   error_str);
+          g_clear_object (&priv->pending_task);
+        }
     }
   else if (priv->pending_task && priv->data->len == 0 && !priv->delete_pending)
     {