fix: nested scan (#6136)

[black-06]

• Do only one thing
• Non breaking API changes
• Tested

What did this pull request do?

fix nested scan panic #6136

User Case Description

var user User
if err := DB.Scopes(func(db *gorm.DB) *gorm.DB {
	var maxID int64
	if err := db.Raw("select max(id) from users").Scan(&maxID).Error; err != nil {
		return db
	}
	return db.Raw("select * from users where id = ?", maxID)
}).Scan(&user).Error; err != nil {
	t.Errorf("failed to find user, got err: %v", err)
}

[a631807682]

This is ok for me, but I think since we implemented a list, it would be a bit wasteful if we can only deal with bools. Of course, I don't think of other scenarios that need to be used at the moment, so I think it's ok.

[jinzhu]

If this PR is only used to fix the panic issue in the nested scan, I think don't panic but return an error should be enough.

We don't expect users to use it like this, for more details: https://gorm.io/docs/method_chaining.html

Going this close this PR for now, feel free to create a new one if those changes are required in other scenarios.

Thank you.