Coroutine pending_increment does not halt execution of coroutine

**Issue by [error454](https://github.com/error454)**
_Wednesday Jul 03, 2019 at 03:02 GMT_
_Originally opened as https://github.com/SkookumScript/SkookumScript-Plugin/issues/7_

----

**UE4 Version**
4.22

**SkookumScript Version**
4.22

**Describe the bug**
Calling `pending_increment` does not suspend a running coroutine as the documentation alludes to.

> // Increments the number of expressions / tasks currently executing in
> // parallel that the invoked object is waiting for a return value from.
> // If the number was 0 and is increased then the invoked object will be
> // suspended until the pending number is reduced back to 0 by calling
> // pending_decrement().

**To Reproduce**
Run the following sample code in the REPL
```javascript
// Coroutine suspend test
!x : 0
!y : 0

!f : (Integer in)[in += 1]
!c1 : branch[loop[f(x) _wait(1.0)]]
!c2 : branch[loop[f(y) _wait(1.0)]]

race
[
loop
[
println(x, ".", y)
_wait(1.0)
]
[
_wait(2)
c1.pending_increment(1)
_wait(1)
c1.pending_decrement
_wait(10)
]
]

// Expected Output:
// 0.0
// 1.1
// 1.2
// 2.3
// ...
```

**Expected behavior**
After calling `c1.pending_increment(1)` the `c1` coroutine should stop running until I call `c1.pending_decrement`.

**Additional context**
I've done some debugging around this. When I break on the call to `c1.pending_increment`, I see that `c1` already has an `m_pending_count` of 1, which means the coroutine is already on the `SkMind`'s pending list. I simply don't see any glue that stops execution of the coroutine once it is on that list.

Reproduce the coroutine behavior in the REPL using the sample and UE4 4.22 setup. Trace pending_increment through SkMind's pending list and inspect how m_pending_count affects execution; done means c1 stops after the increment and resumes after pending_decrement, matching the expected output.

Written by the indexing model from the issue text.