[checklocks]: source file and line number sometimes not printed

### Description

sometimes I see:

`-: return with unexpected locks held (locks: &({param:f}.mu) exclusively)`

notice the usual file name and line number is replaced by `-`.

### Steps to reproduce

```
go: downloading gvisor.dev/gvisor v0.0.0-20241120231458-0add1d20fa0c
```
I haven't been able to extract a simple repro (the fact that I can't tell where the error is happening makes it harder). I happen to believe these are false positive as well but it's hard to prove without knowing what it's unhappy about :-)

### runsc version

```shell

```

### docker version (if using docker)

```shell

```

### uname

_No response_

### kubectl (if using Kubernetes)

```shell

```

### repo state (if built from source)

_No response_

### runsc debug logs (if available)

```shell

```