Fix the length returned by UTF-32 conversion for non-BMP input

Don't optimize the required length as this is a tiny gain resulting in big
problems with the strings containing surrogates for which the actual result is
shorter than the length returned, resulting in extra NUL bytes at the end of
the converted buffer.

This is similar to 3410aa372f (see #16298) but
for UTF-32 and not UTF-16.

Closes #17070.

docs/changes.txt

@@ -211,6 +211,7 @@ wxMSW:
 - Notify shell about the changes done by wxMimeTypesManager (Maarten Bent).
 - Fix wxPrintf() and friends when using MinGW with ANSI stdio option.
 - Fix strike-through support in wxFont with GDI+ (David Vanderson).
+- Fix UTF-32 conversion for non-BMP characters (ARATA Mizuki).
 
 wxOSX/Cocoa: