Fix overflow at return wchar2char (src/backend/utils/adt/pg_locale.c)

Hi,

1. wchar2char has a mistake when checking the return of WideCharToMultiByte
call.
result variable is unsigned, therefore, cannot be less than zero, returning
-1 is not an option.

2. strftime or strftime_win32, return cannot be less than zero.

3. If strftime or strftime_win32, fails, why not abort the loop?

regards,
Ranier Vilela