[mutter] workspace: ensure that workarea data is valid when fetching by monitor num

[Marco Trevisan <marcotrevi src gnome org>]

commit 556136dacc672317f892699f1d989077ffbf24f8
Author: Marco Trevisan (Treviño) <mail 3v1n0 net>
Date:   Thu Oct 12 20:07:12 2017 -0500

    workspace: ensure that workarea data is valid when fetching by monitor num
    
    https://bugzilla.gnome.org/show_bug.cgi?id=788860

 src/core/workspace.c |    7 +++++--
 1 files changed, 5 insertions(+), 2 deletions(-)
---
diff --git a/src/core/workspace.c b/src/core/workspace.c
index f6d3c96..901de45 100644
--- a/src/core/workspace.c
+++ b/src/core/workspace.c
@@ -1099,13 +1099,16 @@ meta_workspace_get_work_area_for_monitor (MetaWorkspace *workspace,
   MetaLogicalMonitor *logical_monitor;
   MetaWorkspaceLogicalMonitorData *data;
 
-  ensure_work_areas_validated (workspace);
-
   logical_monitor =
     meta_monitor_manager_get_logical_monitor_from_number (monitor_manager,
                                                           which_monitor);
+  g_return_if_fail (logical_monitor != NULL);
+
+  ensure_work_areas_validated (workspace);
   data = meta_workspace_get_logical_monitor_data (workspace, logical_monitor);
 
+  g_return_if_fail (data != NULL);
+
   *area = data->logical_monitor_work_area;
 }