[Bug libstdc++/68737] FAIL: 22_locale/num_put/put/char/14220.cc execution test

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68737

--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
And the previous line an alloca call using __len:

        _CharT* __ws = static_cast<_CharT*>(__builtin_alloca(sizeof(_CharT)
                                                             * __len));
        __ctype.widen(__cs, __cs + __len, __ws);


The code assumes __convert_from_v always returns a valid length, but it seems
to be failing and returning -1.