basic_string bug

[Benjamin Kosnik]

Thanks Vadim. This is in, and I checked a slightly modified version of
your testcase into 21_strings/capacity.cc.

Thanks again,
Benjamin

> 1999-06-02  Vadim Egorov  <egorovv@1c.ru>
> 
>         * bits/string.tcc (basic_string::_M_mutate): Fixed memory
>         allocation error