charset: fixed a problem with the global use of the iconv_convenience structure

charset: fixed a problem with the global use of the iconv_convenience structure

We had a crash bug where a cached copy of a iconv convenience pointer
was used after being freed when loadparm asked for iconv to
reload. This could happen if a python module used a iconv based
function before loadparm was completed.

The fix is to ensure that any use of this pointer remains valid, by
reusing the pointer itself when it has already been initialised, but
filling in the child elements with the updated values.