For buffered channel _tryrecv, on success, was unlocking ch._mu too
early - before accessing ch._dataq with ch._dataq.append().

Without the fix, newly added test breaks as e.g.

    golang/golang_test.py::test_chan_buf_recv_vs_tryrecv_race Exception in thread Thread-3:
    Traceback (most recent call last):
      File "/usr/lib/python2.7/threading.py", line 801, in __bootstrap_inner
        self.run()
      File "/usr/lib/python2.7/threading.py", line 754, in run
        self.__target(*self.__args, **self.__kwargs)
      File "/home/kirr/src/tools/go/pygolang-master/golang/golang_test.py", line 317, in _
        assert (_, _rx) == (1, None), ('i%d' % i)
    AssertionError: i30
    assert (0, None) == (1, None)
      At index 0 diff: 0 != 1
      Full diff:
      - (0, None)
      ?  ^
      + (1, None)
      ?  ^