[Bug fortran/65684] Wrong error message when writing to a string

[a.vogt at fulguritus dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65684

--- Comment #9 from Alexander Vogt <a.vogt at fulguritus dot com> ---
Then, it would be as simple as passing generate_error the message instead of
NULL.