在 get 迁移失败的slot 里的key的时候，codis-proxy 异常退出

[guangkun123]

Is this a regression?

Yes

Description

2025/12/10 15:36:05.543345 session.go:82: [INFO] session [0xc0000c4bd0] create: {"ops":0,"create":1765352165,"remote":"127.0.0.1:28855"}
2025/12/10 15:36:09.208985 backend.go:336: [WARN] backend conn [0xc00009c660] to 10.93.214.130:28002, db-0 writer-[0] exit
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x7932f4]

goroutine 43 [running]:
pika/codis/v2/pkg/proxy.(*BackendConn).loopWriter(0xc00009c660, 0x13?)
/data1/codis/pkg/proxy/backend.go:368 +0x374
pika/codis/v2/pkg/proxy.(*BackendConn).run(0xc00009c660)
/data1/codis/pkg/proxy/backend.go:264 +0x1a5
created by pika/codis/v2/pkg/proxy.NewBackendConn
/data1/codis/pkg/proxy/backend.go:55 +0x14b

我codis里有aa、bb、cc、dd、ee、ff、gg

bb 属于slot 942, 我在测试pika关闭slotmigrate后，迁移942 slot

codis 报 [ERROR] Slot[0942]: ERR please open slotmigrate and reload slot

然后我登录proxy，get aa 没问题

get bb 的话。proxy就会崩溃退出。

Please provide a link to a minimal reproduction of the bug

No response

Screenshots or videos

No response

Please provide the version you discovered this bug in (check about page for version information)

pika 版本3.5.6 ， codis是自带的codis ，slot存的是ectd 2.3.7

Anything else?

No response

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Title: When getting the key in the slot where migration failed, codis-proxy exited abnormally.