x11: Keep a reference to the SelectionOutputStream while writing

This ensures close() isn't called from dispose() while we're still busy
writing.

In theory this should never happen, but in practice it just did.

gdk/x11/gdkselectionoutputstream-x11.c

@@ -57,7 +57,7 @@ struct _GdkX11SelectionOutputStreamPrivate {
   GTask *pending_task;
 
   guint incr : 1;
-  guint delete_pending : 1;
+  guint delete_pending : 1; /* owns a reference */
 };
 
 struct _GdkX11PendingSelectionNotify
@@ -292,6 +292,7 @@ gdk_x11_selection_output_stream_perform_flush (GdkX11SelectionOutputStream *stre
       priv->notify = NULL;
     }
 
+  g_object_ref (stream);
   priv->delete_pending = TRUE;
   g_cond_broadcast (&priv->cond);
   g_mutex_unlock (&priv->mutex);
@@ -628,6 +629,7 @@ gdk_x11_selection_output_stream_xevent (GdkDisplay   *display,
       if (gdk_x11_selection_output_stream_needs_flush (stream) &&
           gdk_x11_selection_output_stream_can_flush (stream))
         gdk_x11_selection_output_stream_perform_flush (stream);
+      g_object_unref (stream); /* from unsetting the delete_pending */
       return FALSE;
 
     default: