Fix wx._core.PyAssertionError ... wxGetStockLabel(): invalid stock item ID

[cgohlke]

Using wxPython 3.0.2 on Windows, the mathtext_wx.py example fails:

  File "mathtext_wx.py", line 122, in <module>
    app = MyApp()
  File "X:\Python27-x64\lib\site-packages\wx-3.0-msw\wx\_core.py", line 8628, in __init__
    self._BootstrapApp()
  File "X:\Python27-x64\lib\site-packages\wx-3.0-msw\wx\_core.py", line 8196, in _BootstrapApp
    return _core_.PyApp__BootstrapApp(*args, **kwargs)
  File "mathtext_wx.py", line 117, in OnInit
    frame = CanvasFrame(None, "wxPython mathtext demo app")
  File "mathtext_wx.py", line 65, in __init__
    item = wx.MenuItem(menu, 1000 + i, "")
  File "X:\Python27-x64\lib\site-packages\wx-3.0-msw\wx\_core.py", line 12435, in __init__
    _core_.MenuItem_swiginit(self,_core_.new_MenuItem(*args, **kwargs))
wx._core.PyAssertionError: C++ assertion "Assert failure" failed at ..\..\src\common\stockitem.cpp(213) in wxGetStockLabel(): invalid stock item ID

Apparently, if no or an empty label is given to wx.MenuItem it assumes to use a stock item.

[tacaswell]

Happens on linux too