Use grn_obj_get_value() in grn_expr_exec()

grn_expr_exec() uses grn_obj_get_value_() that returns a pointer to
the value and value size. So we can't get value that is not owned by
object with grn_obj_get_value_(). GRN_COLUMN_INDEX returns estimate
size as value and it is not owned value. In other words, we can't use
grn_obj_get_value_() for GRN_COLUMN_INDEX.

grn_obj_get_value() writes value to the passed grn_obj. So
grn_obj_get_value() can returns not owned value.

grn_obj_get_value_() is lighter API rather than grn_obj_get_value()
because grn_obj_get_value_() is just returns a pointer. It doesn't
write the value. Maybe, it is the reason why grn_expr_exec() uses
grn_obj_get_value_().

But grn_expr_exec() always writes value returned from
grn_obj_get_value_(). There is no speed merit. So we can use
grn_obj_get_value() rather than grn_obj_get_value_() without speed
down.

We can get value of GRN_COLUMN_INDEX by using grn_obj_get_value().