MDEV-10386 Assertion `fixed == 1' failed in virtual String* Item_func_conv_charset::val_str(String*)

The patch b96c196f added a new call for
safe_charset_converter() without a corresponding fix_fields().
In case of a sub-query the created Item remained in non-fixed state.
The problem did not show up with literal derived expressions, only
subselects were affected. This patch adds a corresponding fix_fields()
to the previously added safe_charset_converter().