Fix buffer overflow reported by the compiler:

wxChar m_ext[kMacExtensionMaxLength] ;
...
m_ext[kMacExtensionMaxLength] = 0 ; //warning: array index 16 is past the end of the array (which contains 16 elements) [-Warray-bounds]

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_2_8_BRANCH@76603 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775