gh-106844: Fix issues in _winapi.LCMapStringEx (GH-107832)

* Strings with length from 2**31-1 to 2**32-2 always caused MemoryError,
   it doesn't matter how much memory is available.
* Strings with length exactly 2**32-1 caused OSError.
* Strings longer than 2**32-1 characters were truncated due to integer overflow bug.
* Strings containing the null character were truncated at the first null character.

Now strings longer than 2**31-1 characters caused OverflowError and the null character is allowed.