[mutter] selection: Fix task leak in error case of source_read_cb

[Robert Mader <rmader src gnome org>]

commit e7928ce6eed69475fdf537d8c8975e21d57757d8
Author: Sebastian Keller <skeller gnome org>
Date:   Thu Jan 30 10:35:01 2020 +0100

    selection: Fix task leak in error case of source_read_cb
    
    Fixes the last part of https://gitlab.gnome.org/GNOME/mutter/issues/1006
    https://gitlab.gnome.org/GNOME/mutter/merge_requests/1001

 src/core/meta-selection.c | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/src/core/meta-selection.c b/src/core/meta-selection.c
index e0ff9f4b1..7e86c5269 100644
--- a/src/core/meta-selection.c
+++ b/src/core/meta-selection.c
@@ -270,6 +270,7 @@ source_read_cb (MetaSelectionSource *source,
   if (!stream)
     {
       g_task_return_error (task, error);
+      g_object_unref (task);
       return;
     }