Fixes crash in PanelLayoutManager

During shutdown of sysui I was occasionally crashing in
PanelLayoutManager as the window was null.

BUG=594600
TEST=none
R=jamescook@chromium.org

Review-Url: https://codereview.chromium.org/2108383002
Cr-Commit-Position: refs/heads/master@{#403020}

ash/common/wm/panels/panel_layout_manager.cc

@@ -572,7 +572,7 @@ void PanelLayoutManager::RestorePanel(WmWindow* panel) {
 }
 
 void PanelLayoutManager::Relayout() {
-  if (!shelf_)
+  if (!shelf_ || !shelf_->GetWindow())
     return;
 
   // Suppress layouts during overview mode because changing window bounds