bfd: correct dir separator conversion for Win32

bfd: correct dir separator conversion for Win32

Iterating a wchar_t array holding the conversion of multi-byte (likely
UTF-8) input using array indexes from the corresponding char array isn't
going to work as soon as any characters wider than a single char are
present. Simply walk the wchar_t array all by itself.

While looking at that code I also noticed a wrong argument being passed to
a later MultiByteToWideChar() invocation: This needs to be number of
characters, which isn't sizeof() when the array is of wchar_t elements.