Fixes #5532 win async write (#5791)

* nimgrab tool bugfix: don't divide by zero
* fixes #5532 (asyncfile write on Windows)
* add a comment about what has been tried instead

tests/async/tasyncfilewrite.nim

@@ -0,0 +1,17 @@
+discard """
+  output: '''string 1
+string 2
+string 3'''
+"""
+# bug #5532
+import os, asyncfile, asyncdispatch
+
+removeFile("test.txt")
+let f = openAsync("test.txt", fmWrite)
+var futs = newSeq[Future[void]]()
+for i in 1..3:
+  futs.add(f.write("string " & $i & "\n"))
+waitFor(all(futs))
+f.close()
+echo readFile("test.txt")
+