bpo-33706: Fix pymain_parse_cmdline_impl() (GH-7283)

Fix a crash in Python initialization when parsing the command line
options.

Fix memcpy() size parameter: previously, we read one wchar_t after
the end of _PyOS_optarg. Moreover, don't copy the trailingg NUL
character: we write it manually anyway.

Thanks Christoph Gohlke for the bug report and the fix!