os/user: `bad pointer in frame os/user.lookupUnixGid`

```
#!watchflakes
post <- `bad pointer in frame os/user.lookupUnixGid`
```

(extracted from https://github.com/golang/go/issues/55167#issuecomment-1342643031)

I believe this is related to the `os/user` allocation changes in [CL 449335](https://go.dev/cl/449335) and/or [CL 449316](https://go.dev/cl/449316). (CC @ianlancetaylor @rsc)