cros: Fix clusterfuzz crash opening stylus palette

Clusterfuzz skips the login pathway and injects keystrokes immediately on
startup. This can trigger accelerators before the status area is initialized.
Add a null check -- this case doesn't happen in production.

BUG=702480
TEST=clusterfuzz

Review-Url: https://codereview.chromium.org/2758863002
Cr-Commit-Position: refs/heads/master@{#457827}

ash/common/accelerators/accelerator_controller.cc

@@ -514,9 +514,11 @@ void HandleShowStylusTools() {
 
   RootWindowController* root_window_controller =
       Shell::GetWmRootWindowForNewWindows()->GetRootWindowController();
-  PaletteTray* palette_tray =
-      root_window_controller->GetShelf()->GetStatusAreaWidget()->palette_tray();
-  palette_tray->ShowPalette();
+  StatusAreaWidget* status_area_widget =
+      root_window_controller->GetShelf()->GetStatusAreaWidget();
+  // Tests (clusterfuzz) can trigger this before the status area is ready.
+  if (status_area_widget)
+    status_area_widget->palette_tray()->ShowPalette();
 }
 
 bool CanHandleShowStylusTools() {