server/process: Avoid tracebacks at exit
    
    In theory this should have been worked around but is still occurring. Add
    it to the list of things to ignore when bitbake is shutting down.
    
    Traceback (most recent call last):
      File "/usr/lib64/python3.9/threading.py", line 973, in _bootstrap_inner
        self.run()
      File "/home/pokybuild/yocto-worker/oe-selftest-fedora/build/bitbake/lib/bb/server/process.py", line 698, in startCallbackHandler
        event = self.reader.get()
      File "/home/pokybuild/yocto-worker/oe-selftest-fedora/build/bitbake/lib/bb/server/process.py", line 722, in get
        res = self.reader.recv_bytes()
      File "/usr/lib64/python3.9/multiprocessing/connection.py", line 221, in recv_bytes
        buf = self._recv_bytes(maxlength)
      File "/usr/lib64/python3.9/multiprocessing/connection.py", line 426, in _recv_bytes
        return self._recv(size)
      File "/usr/lib64/python3.9/multiprocessing/connection.py", line 384, in _recv
        chunk = read(handle, remaining)
    TypeError: an integer is required (got type NoneType)'
    
    Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>