Panic if type name "Subscription"

### What happened?
A panic occurs if define type of Subscription
```
type Subscription implements Node {
id: ID!
userID: ID!
name: String!
deadline: Int!
createdAt: Int!
updatedAt: Int!
}

type Query {
findSubscription(name:String!): Subscription
}
```
```
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x18 pc=0x130447d]

goroutine 1 [running]:
github.com/99designs/gqlgen/codegen/config.(*TypeReference).IsPtr(...)
/Users/foo/go/pkg/mod/github.com/99designs/gqlgen@v0.10.2/codegen/config/binder.go:255
github.com/99designs/gqlgen/codegen.(*builder).buildField(0xc0002dd5d0, 0xc000627600, 0xc000167c00, 0x0, 0x1b189e0, 0x0)
/Users/foo/go/pkg/mod/github.com/99designs/gqlgen@v0.10.2/codegen/field.go:70 +0x35d
github.com/99designs/gqlgen/codegen.(*builder).buildObject(0xc0002dd5d0, 0xc0001d6900, 0x1618581, 0x6, 0x0)
/Users/foo/go/pkg/mod/github.com/99designs/gqlgen@v0.10.2/codegen/object.go:73 +0x4ec
github.com/99designs/gqlgen/codegen.BuildData(0xc0001b8000, 0x16fbc60, 0x1b17d80, 0x0)
/Users/foo/go/pkg/mod/github.com/99designs/gqlgen@v0.10.2/codegen/data.go:89 +0x4a7
github.com/99designs/gqlgen/api.Generate(0xc0001b8000, 0x0, 0x0, 0x0, 0x0, 0xc0000a2006)
/Users/foo/go/pkg/mod/github.com/99designs/gqlgen@v0.10.2/api/generate.go:41 +0x276
github.com/99designs/gqlgen/cmd.glob..func1(0xc0001b6000)
/Users/foo/go/pkg/mod/github.com/99designs/gqlgen@v0.10.2/cmd/gen.go:39 +0xa2
github.com/urfave/cli.HandleAction(0x1553e60, 0x1659b50, 0xc0001b6000, 0xc0001b6000, 0x0)
/Users/foo/go/pkg/mod/github.com/urfave/cli@v1.22.2/app.go:525 +0x127
github.com/urfave/cli.Command.Run(0x161d37a, 0x8, 0x0, 0x0, 0x0, 0x0, 0x0, 0x1649e4f, 0x29, 0x0, ...)
/Users/foo/go/pkg/mod/github.com/urfave/cli@v1.22.2/command.go:174 +0x521
github.com/urfave/cli.(*App).Run(0xc000102540, 0xc00000e080, 0x2, 0x2, 0x0, 0x0)
/Users/foo/go/pkg/mod/github.com/urfave/cli@v1.22.2/app.go:276 +0x729
github.com/99designs/gqlgen/cmd.Execute()
/Users/foo/go/pkg/mod/github.com/99designs/gqlgen@v0.10.2/cmd/root.go:40 +0x224
main.main()
/Users/foo/go/pkg/mod/github.com/99designs/gqlgen@v0.10.2/main.go:8 +0x20
```
### What did you expect?
No Panic and generate code

### Minimal graphql.schema and models to reproduce
```
type Subscription {
id: ID!
userID: ID!
createdAt: Int!
updatedAt: Int!
}
```
### versions
- `gqlgen version`?
v0.10.2
- `go version`?
go version go1.12.9 darwin/amd64
- dep or go modules?
go modules