Implements `asyncdispatch.getIoHandler` and assert on nil futures.

lib/pure/asyncdispatch.nim

@@ -262,6 +262,11 @@ when defined(windows) or defined(nimdoc):
       setGlobalDispatcher(newDispatcher())
     result = gDisp
 
+  proc getIoHandler*(disp: PDispatcher): Handle =
+    ## Returns the underlying IO Completion Port handle (Windows) or selector
+    ## (Unix) for the specified dispatcher.
+    return disp.ioPort
+
   proc register*(fd: cint | SocketHandle | AsyncFD): AsyncFD {.discardable.} =
     ## Registers ``fd`` with the dispatcher.
     ##
@@ -1103,6 +1108,9 @@ else:
       setGlobalDispatcher(newDispatcher())
     result = gDisp
 
+  proc getIoHandler*(disp: PDispatcher): Selector[AsyncData] =
+    return disp.selector
+
   proc register*(fd: cint | SocketHandle | AsyncFD): AsyncFD {.discardable.} =
     let p = getGlobalDispatcher()
     when fd is AsyncFD:

lib/pure/asyncfutures.nim

@@ -281,6 +281,7 @@ proc asyncCheck*[T](future: Future[T]) =
   ## finished with an error.
   ##
   ## This should be used instead of ``discard`` to discard void futures.
+  assert(not future.isNil, "Future is nil")
   future.callback =
     proc () =
       if future.failed: