sendTo: Exception message: index out of bounds
```nim
import asyncdispatch2

const HELLO_PORT = 1900
const TEST_MSG = "testmsg"
const MSG_LEN = TEST_MSG.len()

var udp4DataAvailable: DatagramCallback = proc(transp: DatagramTransport, remote: TransportAddress): Future[void] {.async, gcsafe.} =
  echo "Data from:", remote

proc main(): Future[void] {.async.} =
  var myself = initTAddress("127.0.0.1:" & $HELLO_PORT)
  var data: ref byte
  data = new byte  
  var dsock4 = newDatagramTransport[byte](udp4DataAvailable, udata = data, local = myself)
  await dsock4.sendTo(myself, TEST_MSG, MSG_LEN)

asyncCheck main()
while true:
  poll()

# Fails with
# asyncdispatch2Bug.nim(19) asyncdispatch2Bug
# asyncmacro2.nim(306)     main
# asyncmacro2.nim(36)      main_continue
# asyncdispatch2Bug.nim(17) mainIter
# datagram.nim(622)        sendTo
# system.nim(2807)         sysFatal
# [[reraised from:
# asyncdispatch2Bug.nim(21) asyncdispatch2Bug
# asyncloop.nim(226)       poll
# asyncfutures2.nim(370)   cb
# ]]
# Error: unhandled exception: index out of bounds
# Async traceback:
#   asyncdispatch2Bug.nim(19) asyncdispatch2Bug
#   asyncmacro2.nim(306)      main
#   asyncmacro2.nim(36)       main_continue
#   asyncdispatch2Bug.nim(17) mainIter
#   datagram.nim(622)         sendTo
#   system.nim(2807)          sysFatal
# Exception message: index out of bounds
# Exception type: [IndexError]
```
If i `sendTo` from the callback it works fine.
Do i maybe use it wrong?
