Fix NFD_PickFolder() asserting in debug with non ASCII default path

[blep]

• How to reproduce:
  Call NFD_PickFolder() passing a non ASCII path for defaultPath on Windows with Visual Studio 2019, Win32 64 bits build.

• Description:
  The sanity check in CopyNFDCharToWChar() assert (called by SetDefaultPath()):

#ifdef _DEBUG
    int inStrCharacterCount = static_cast<int>(NFDi_UTF8_Strlen(inStr));
    assert( ret == inStrCharacterCount );
#else

• Cause:
  This is caused by right shifting char which is negative, which is implementation defined behavior. I fixed this by casting to a unsigned value before shifting.

I was only able to test this in my own build env (VS 2019), so make sure to run it through your private build bots.