MDEV-32879 Server crash in my_decimal::operator= or unexpected ER_DUP_ENTRY upon comparison with INET6 and similar types

During the 10.5->10.6 merge please use the 10.6 code on conflicts.

This is the 10.5 version of the patch (a backport of the 10.6 version).
Unlike 10.6 version, it makes changes in plugin/type_inet/sql_type_inet.*
rather than in sql/sql_type_fixedbin.h

Item_bool_rowready_func2, Item_func_between, Item_func_in
did not check if a not-NULL argument of an arbitrary data type
can produce a NULL value on conversion to INET6.

This caused a crash on DBUG_ASSERT() in conversion failures,
because the function returned SQL NULL for something that
has Item::maybe_null() equal to false.

Adding setting NULL-ability in such cases.

Details:

- Removing the code in Item_func::setup_args_and_comparator()
  performing character set aggregation with optional narrowing.
  This aggregation is done inside Arg_comparator::set_cmp_func_string().
  So this code was redundant

- Removing Item_func::setup_args_and_comparator() as it git simplified to
  just to two lines:
    convert_const_compared_to_int_field(thd);
    return cmp->set_cmp_func(thd, this, &args[0], &args[1], true);
  Using these lines directly in:
    - Item_bool_rowready_func2::fix_length_and_dec()
    - Item_func_nullif::fix_length_and_dec()

- Adding a new virtual method:
  - Type_handler::Item_bool_rowready_func2_fix_length_and_dec().

- Adding tests detecting if the data type conversion can return SQL NULL into
  the following methods of Type_handler_inet6:
  - Item_bool_rowready_func2_fix_length_and_dec
  - Item_func_between_fix_length_and_dec
  - Item_func_in_fix_comparator_compatible_types

sql/sql_type.cc

@@ -5675,6 +5675,14 @@ Type_handler_string_result::Item_func_hybrid_field_type_get_date(
 
 /***************************************************************************/
 
+bool Type_handler::Item_bool_rowready_func2_fix_length_and_dec(THD *thd,
+                                          Item_bool_rowready_func2 *func) const
+{
+  return func->fix_length_and_dec_generic(thd, this);
+}
+
+/***************************************************************************/
+
 bool Type_handler_numeric::
        Item_func_between_fix_length_and_dec(Item_func_between *func) const
 {