[panic] db.Transaction panic after close()

[zzjin]

GORM Playground Link

go-gorm/playground#104

Description

when use db.Transaction on unexpect state such as call after DB().Close() panic.
case: maybe call direct to sql/mysql Close() cannot clean some gorm spec states.

当在某些其他位置数据库被关闭之后再执行db.Transaction直接崩溃
可能的原因:现在直接暴露关闭原生的sql的Close()方法可能导致了gorm自己的内部状态的不一致