fixes async regression

tests/async/tnimcall_to_closure.nim

@@ -0,0 +1,17 @@
+
+import asyncdispatch
+
+proc defaultOnProgressChanged() = discard
+
+proc ask(x: proc()) = x()
+
+proc retrFile*(onProgressChanged: proc() {.nimcall.}): Future[void] =
+  var retFuture = newFuture[void]("retrFile")
+  iterator retrFileIter(): FutureBase {.closure.} =
+    ask(onProgressChanged)
+    complete(retFuture)
+
+  var nameIterVar = retrFileIter
+  return retFuture
+
+discard retrFile(defaultOnProgressChanged)