ITEM_PARAM::SAFE_CHARSET_CONVERTER

ISSUE:
------
Charset conversion on a null parameter is not handled
correctly.

SOLUTION:
---------
Item_param's charset converter does not handle the case
where it might have to deal with a null value. This is
fine for other charset converters since the value is not
supplied to them at runtime.

The fix is to check if the parameter is now set to null and
return an Item_null object. Also, there is no need to
initialize Item_param's cnvitem in the constructor to a
string. This can be done in
ITEM_PARAM::SAFE_CHARSET_CONVERTER itself.

Members of Item_param, cnvbuf and cnvstr, have been removed
and cnvitem has been made a local variable in
ITEM_PARAM::SAFE_CHARSET_CONVERTER.