Fix removeReader/removeWriter can produce SIGSEGV.

asyncdispatch2.nimble

@@ -1,5 +1,5 @@
 packageName   = "asyncdispatch2"
-version       = "2.1.5"
+version       = "2.1.6"
 author        = "Status Research & Development GmbH"
 description   = "Asyncdispatch2"
 license       = "Apache License 2.0 or MIT"

asyncdispatch2/asyncloop.nim

@@ -234,7 +234,8 @@ template processCallbacks(loop: untyped) =
     # can be generated.
     if len(loop.callbacks) == 0: break
     let callable = loop.callbacks.popFirst()
-    callable.function(callable.udata)
+    if not isNil(callable.function):
+      callable.function(callable.udata)
 
 when defined(windows) or defined(nimdoc):
   import winlean, sets, hashes