Fix race condition in CommandScheduler

[chros73]

Patched by @pyroscope.

Relates to: #515

[rakshasa]

This isn't the fix you are looking for.

It doesn't address the race condition as the scheduler isn't a thread-safe container, and at best you hide the issue a bit.

The scheduler is only ever supposed to be called while in the main thread, or updated while the global lock is being held by a particular thread.

If there's an issue it might be due to the worker thread starting in main.cc before the main thread.

[chros73]

It doesn't address the race condition ... If there's an issue it might be due to the worker thread starting in main.cc before the main thread.

Ok, thanks for the explanation.
I modified "Closes" to "Relates to" in the description.

... at best you hide the issue a bit.

Well, it prevents crashing. You can modify the title of this pull request if you still feel that this is useful.

[rakshasa]

To debug this we probably should add a check to see if the thread calling the command holds the global lock.