umqtt.simple check_msg() leaves socket in nonblocking state

[peterhinch]

This causes failure in a subsequent call to subscribe() as reported here. The following (tested) fix is one option:

    def check_msg(self):
        self.sock.setblocking(False)
        result = self.wait_msg()
        self.sock.setblocking(True)
        return result

[pfalcon]

Thanks for diagnosing. And a stupid think on m part of course, fixed as was intended in 4a8ef9f