asyncio: don't do anything in _SelectorDatagramTransport if we're closed

[istankovic]

It could happen that the transport's sendto() method is invoked after
the transport was closed. Closing the transport sets the _sock attribute
to None, which then causes the following exception:

Traceback (most recent call last):
  File "/usr/lib/python3.9/asyncio/selector_events.py", line 1056, in sendto
self._sock.sendto(data, addr)
AttributeError: 'NoneType' object has no attribute 'sendto'

So just check if we're closed before doing anything, similarly to what we already
do in other methods.

[the-knights-who-say-ni]

Hello, and thanks for your contribution!

I'm a bot set up to make sure that the project can legally accept this contribution by verifying everyone involved has signed the PSF contributor agreement (CLA).

Recognized GitHub username

We couldn't find a bugs.python.org (b.p.o) account corresponding to the following GitHub usernames:

@istankovic

This might be simply due to a missing "GitHub Name" entry in one's b.p.o account settings. This is necessary for legal reasons before we can look at this contribution. Please follow the steps outlined in the CPython devguide to rectify this issue.

You can check yourself to see if the CLA has been received.

Thanks again for the contribution, we look forward to reviewing it!

[github-actions]

This PR is stale because it has been open for 30 days with no activity.

[github-actions]

Closing this stale PR because the CLA is still not signed.