Make mojo_shell_child not crash when done

mojo/runner/child_process.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "mojo/runner/child_process.h"
 
 #include "base/base_switches.h"
 #include "base/bind.h"
 #include "base/callback_helpers.h"
 #include "base/command_line.h"
@@ skipping 48 matching lines @@
 
     // Copy and assign allowed.
   };
 
   Blocker() : event_(true, false) {}
   ~Blocker() {}
 
   void Block() {
     DCHECK(run_after_.is_null());
     event_.Wait();
-    run_after_.Run();
+    if (!run_after_.is_null())
+      run_after_.Run();
   }
 
   Unblocker GetUnblocker() { return Unblocker(this); }
 
  private:
   base::WaitableEvent event_;
   base::Closure run_after_;
 
   DISALLOW_COPY_AND_ASSIGN(Blocker);
 };
@@ skipping 218 matching lines @@
   // This will block, then run whatever the controller wants.
   blocker.Block();
 
   app_context.Shutdown();
 
   return 0;
 }
 
 }  // namespace runner
 }  // namespace mojo