`rc` in `nt_try_fast_exec` should be set to 1 as default.

First of all, let me thank you for your great software.

`rc` in `nt_try_fast_exec` should be set to 1 as default so that "Command not found." can be shown correctly.
This modification affects only Windows.

If `rc` is not updated in `do while` loop, `nt_try_fast_exec` returns 0, which means "command executed successfully".
i.e. If `bit_extern` returns 0 for all index `i`, `rc` is not updated in `do while` and `nt_try_fast_exec` returns 0.

Test case:
```
% setenv PATH C:/Windows
% rehash
% a
 => "Command not found." is not shown.
```

If my patch is pointless, let me know please.

Regards,
Murase