Strange issue with new Scheduler implementation

[rh101]

@halx99 I've been getting random crashes in Scheduler::update() (Axmol 2.x), so far only while debugging. It's only happened after hitting a breakpoint, then continuing application execution, which instantly crashes at the following:

// updates with priority == 0
for (auto&& entry : _updates0List)
{
    // *** Crash occurs here, because "entry" is invalid ***
    if ((!entry->paused) && (!entry->markedForDeletion))
    {
        entry->callback(dt); 
    }
}

I've never seen this error prior to the changes made to the Scheduler recently, and I can't easily reproduce the issue, which is what is stopping me from being able to create a simple test case.

The only thing I can think of is that there may be code affecting the scheduler in the callback made by entry->callback(dt);, which somehow results in an invalid entry within _updates0List. Is there any way that would happen with the new Scheduler implementation?

I won't be too concerned about this issue unless it occurs under normal conditions without the debugger.

[rh101]

Issue #1391