[gfortran] Fix PR 16579: Signed/Unsigned confusion in ICHAR

[Tobias Schlüter]

Tobias Schlüter wrote:
> -  type = gfc_typenode_for_spec (&expr->ts);
> +  type = gfc_get_int_type (gfc_default_integer_kind);
...
> +  se->expr = fold_convert (type,
> +                          fold_convert(unsigned_char_type_node, se->expr));

I copy/pasted this to early, I added a minor cleanup: the actual patch
replaces type in the second line by its value and removes the then unused
variable.

- Tobi