Make mojo_shell_child not crash when done

This is https://codereview.chromium.org/1082973004 ported to chrome.

BUG=none
TEST=none
R=viettrungluu@chromium.org

Review URL: https://codereview.chromium.org/1139973002

Cr-Commit-Position: refs/heads/master@{#329513}

mojo/runner/child_process.cc

@@ -66,7 +66,8 @@ class 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); }