Bug: ntpq shell escape traceback

ntpq> !
Traceback (most recent call last):
  File "./ntpq/ntpq", line 1636, in <module>
    interpreter.cmdloop()
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/cmd.py", line 142, in cmdloop
    stop = self.onecmd(line)
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/cmd.py", line 210, in onecmd
    return self.default(line)
  File "./ntpq/ntpq", line 187, in default
    dotext = 'do_'+cmd
TypeError: cannot concatenate 'str' and 'NoneType' objects

We don't implement do_shell, so we just need to catch the None returned from parseline() gracefully.