openapi v3 petstore does not work

I tried to run the following command to generate petstore.pb file
```sh
▶ gnostic --pb-out=. examples/v3.0/json/petstore.json
```
Then run report for the `petstore.pb` file. it gave

```sh
▶ report petstore.pb
panic: unexpected EOF

goroutine 1 [running]:
main.readDocumentFromFileWithName(0x7fff5fbff13f, 0xb, 0xc420098360)
/Users/xxx/.gvm/pkgsets/go1.10/api/src/github.com/googleapis/gnostic/apps/report/main.go:40 +0x14d
main.main()
/Users/xxx/.gvm/pkgsets/go1.10/api/src/github.com/googleapis/gnostic/apps/report/main.go:232 +0x98

```