Make mojo_shell_child not crash when done.

BUG=Fixes #96
R=qsr@chromium.org

Review URL: https://codereview.chromium.org/1082973004
diff --git a/shell/child_main.cc b/shell/child_main.cc
index 4bab35e..6fc14b3 100644
--- a/shell/child_main.cc
+++ b/shell/child_main.cc
@@ -65,7 +65,8 @@
   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); }