Clipboard test failure on windows
Intermittent, seen on 2.7 64bits

``` python
======================================================================
ERROR: test_round_trip_frame_string (classname)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "c:\Python27-AMD64\Lib\unittest\case.py", line 331, in run
    testMethod()
  File "C:\workspace\pandas_tests\BITS\64\PYTHONVER\27\pandas\io\tests\test_clipboard.py", line 63, in test_round_trip_frame_string
    self.check_round_trip_frame(dt,excel=False)
  File "C:\workspace\pandas_tests\BITS\64\PYTHONVER\27\pandas\io\tests\test_clipboard.py", line 50, in check_round_trip_frame
    data.to_clipboard(excel=excel, sep=sep)
  File "C:\workspace\pandas_tests\BITS\64\PYTHONVER\27\pandas\core\generic.py", line 934, in to_clipboard
    clipboard.to_clipboard(self, excel=excel, sep=sep, **kwargs)
  File "C:\workspace\pandas_tests\BITS\64\PYTHONVER\27\pandas\io\clipboard.py", line 76, in to_clipboard
    clipboard_set(objstr)
  File "C:\workspace\pandas_tests\BITS\64\PYTHONVER\27\pandas\util\clipboard.py", line 72, in winSetClipboard
    ctypes.cdll.msvcrt.strcpy(ctypes.c_char_p(pchData), bytes(text))
WindowsError: exception: access violation writing 0x0000000000000000
```
