Signed/unsigned comparison warning on FreeBSD

[historic_bruno]

This warning occurs almost constantly when building on FreeBSD:

../../../source/lib/os_path.h: In function 'std::string OsString(const OsPath&)':
../../../source/lib/os_path.h:48: warning: comparison between signed and unsigned integer expressions

I checked the system headers and UCHAR_MAX is indeed defined as 0xffU, but it seems wchar_t is actually an int on FreeBSD.

[historic_bruno]

I didn't pay attention to what the code is actually doing, it's checking if the wchar_t value could fit in an unsigned char.

[philip]

In 11251:

Fix #1140 (signedness warning on some platforms).