If no package is declared in the proto file. Will get a nil pointer error

If no package is declared in the proto file. 
Will get a nil pointer error on 
```go
ns := &ns{
		Package:   *file.Package, // <-- this line
		Namespace: namespace(file.Package, "\\"),
		Import:    make(map[string]string),
	}
```
should we throw a more verbose exception to warn the user?