validation: fix nil dereference when handling multierror in hooks_stdin
validation: fix nil dereference when handling multierror in hooks_stdin

As `stdinStateCheck()` can return nil for errs, we need to access to
`errs.Error()` only when errs is not nil. So conditionally compose
newError for both cases, errs == nil and errs != nil, to avoid panics
like that:

```
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x18 pc=0x4f5a02]

goroutine 1 [running]:
github.com/opencontainers/runtime-tools/vendor/github.com/hashicorp/go-multierror.(*Error).Error(0x0, 0x7dfc20, 0xc420224480)
	.../github.com/opencontainers/runtime-tools/vendor/github.com/hashicorp/go-multierror/multierror.go:15 +0x22
main.main()
	.../github.com/opencontainers/runtime-tools/validation/hooks_stdin.go:154 +0x11ba
```

Signed-off-by: Dongsu Park <696fb7639d678a1e692fa501217d7509545d7331@kinvolk.io>

opencontainers/runtime-tools,opencontainers/runtime-tools,opencontainers/ocitools,opencontainers/ocitools