bpo-35766 follow-up: Add an error check to new_type_comment()

[gvanrossum]

If PyUnicode_DecodeUTF8() were to return NULL, PyArena_AddPyObject() would crash.
Found by @msullivan for python/typed_ast#93.

https://bugs.python.org/issue35766

[bedevere-bot]

@gvanrossum: Please replace # with GH- in the commit message next time. Thanks!

[bedevere-bot]

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot AMD64 FreeBSD CURRENT Shared 3.x has failed when building commit 4b250fc.

What do you need to do:

1. Don't panic.
2. Check the buildbot page in the devguide if you don't know what the buildbots are or how they work.
3. Go to the page of the buildbot that failed (https://buildbot.python.org/all/#builders/168/builds/562) and take a look at the build logs.
4. Check if the failure is related to this commit (4b250fc) or if it is a false positive.
5. If the failure is related to this commit, please, reflect that on the issue and make a new Pull Request with a fix.

You can take a look at the buildbot page here:

https://buildbot.python.org/all/#builders/168/builds/562

Click to see traceback logs

Traceback (most recent call last):
  File "/usr/home/buildbot/python/3.x.koobs-freebsd-current/build/Lib/multiprocessing/process.py", line 302, in _bootstrap
    self.run()
  File "/usr/home/buildbot/python/3.x.koobs-freebsd-current/build/Lib/multiprocessing/process.py", line 99, in run
    self._target(*self._args, **self._kwargs)
  File "/usr/home/buildbot/python/3.x.koobs-freebsd-current/build/Lib/multiprocessing/managers.py", line 572, in _run_server
    writer.send(server.address)
  File "/usr/home/buildbot/python/3.x.koobs-freebsd-current/build/Lib/multiprocessing/connection.py", line 206, in send
    self._send_bytes(_ForkingPickler.dumps(obj))
  File "/usr/home/buildbot/python/3.x.koobs-freebsd-current/build/Lib/multiprocessing/connection.py", line 411, in _send_bytes
    self._send(header + buf)
  File "/usr/home/buildbot/python/3.x.koobs-freebsd-current/build/Lib/multiprocessing/connection.py", line 368, in _send
    n = write(self._handle, buf)
BrokenPipeError: [Errno 32] Broken pipe
/usr/home/buildbot/python/3.x.koobs-freebsd-current/build/Lib/multiprocessing/process.py:318: ResourceWarning: unclosed <socket.socket fd=5, family=AddressFamily.AF_INET, type=SocketKind.SOCK_STREAM, proto=0, laddr=('127.0.0.1', 19024)>
  traceback.print_exc()
ResourceWarning: Enable tracemalloc to get the object allocation traceback

[gvanrossum]

I believe the buildbot failure is a false positive.