Jester crashes when async sleeping
As described in the title. Here is the minimized code:
```nim
import jester, asyncdispatch, random

routes:
  get "/":
    await sleepAsync(rand(2000))
    resp "hi"
```
and here is the stacktrace
```
main.nim(3)              main
jester.nim(498)          serve
httpbeast.nim(540)       run
httpbeast.nim(371)       eventLoop
httpbeast.nim(227)       processEvents
asyncdispatch.nim(1653)  poll
asyncdispatch.nim(1394)  runOnce
asyncdispatch.nim(234)   processPendingCallbacks
asyncmacro.nim(38)       handleRequestSlowNimAsyncContinue
[[reraised from:
main.nim(3)              main
jester.nim(498)          serve
httpbeast.nim(540)       run
httpbeast.nim(371)       eventLoop
httpbeast.nim(227)       processEvents
asyncdispatch.nim(1653)  poll
asyncdispatch.nim(1394)  runOnce
asyncdispatch.nim(234)   processPendingCallbacks
asyncfutures.nim(262)    :anonymous
httpbeast.nim(287)       :anonymous
httpbeast.nim(148)       onRequestFutureComplete
]]
Error: unhandled exception: Async procedure (handleRequestSlow) yielded `nil`, are you await'ing a `nil` Future? [AssertionDefect]
```
```
Nim Compiler Version 1.6.4 [Linux: amd64]
Compiled at 2022-02-09
Copyright (c) 2006-2021 by Andreas Rumpf
Fedora 35
```
Error goes away after disabling assertions with seemingly no side effects, though that's basically a hack. Note that this fails non-deterministically - I've been testing it with wrk, though in my non-minimized code it fails after a couple of requests from a normal web browser.

```
wrk -c 100 -t 4 -d 10 http://localhost:5000/
```

I've tested this exact code with Prologue and it didn't have any issues, though it did have me annotate the callback proc with `.gcsafe.`
Not sure if that rules out httpbeast completely (as Prologue uses a fork).

The non-minimized code isn't actually doing an async sleep with random wait, it's doing a web request. I thought the web request was causing the issue until I realized I could replicate it with a similar wait time by using sleep. Hopefully this is enough information for at least someone else to reproduce!  