We have made utf32 superset of all charsets, and changed Arg_comparator::
set_cmp_func() to allow compare function pick superset charset to do
comparison. But agg_item_set_converter() is only used to check the charset
of one operand of comparing clause. This causes binary string which can be
of any number of bytes is passed to my_strnncollsp_utf32_bin(), and makes
the assertion fail.

Fix:
agg_item_set_converter() is used on both operands, so it can add converter
if any operand's charset needs conversion.

Change-Id: Iba31a1030c48d904ee9cbab563fb6459fb7ae422