(PUP-391) Fix INVALID_FILE_ATTRIBUTES on x64

• The GetFileAttributes Win32 API call returns a DWORD, which is a platform independent 32-bit wide unsigned integer: http://msdn.microsoft.com/en-us/library/windows/desktop/aa364944(v=vs.85).aspx It's max value is 4294967295: http://msdn.microsoft.com/en-us/library/windows/desktop/aa383751(v=vs.85).aspx
• FFI::Pointer.new(-1).address is platform dependent and on x64 is 18446744073709551615, the max value of a 64-bit wide unsigned int.
• The INVALID_FILE_ATTRIBUTES constant is a valid return value of GetFileAttributes and is defined in winbase.h as: #define INVALID_FILE_ATTRIBUTES ((DWORD)-1)
• Therefore INVALID_FILE_ATTRIBUTES should be defined as 0xFFFFFFFF