* libc/stdlib/mbrtowc.c (_mbrtowc_r): Fix case where s is null
pointer to match C99 spec.
* libc/stdlib/mbsrtowcs.c (_mbsrtowc_r): Fix to ignore len when
dst is NULL. Also fix to not alter src pointer when dst is NULL
and call _mbrtowc_r instead of _mbtowc_r.
* libc/stdlib/wcsrtombs.c (_wcsrtombs_r): Call _wcrtomb_r instead
of _wctomb_r.
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