bad union syntax = parser non-termination

there is an infinite loop in the parser. here's a reproduction:

```
package main

import "github.com/graph-gophers/graphql-go"

func main() {
	graphql.MustParseSchema(`type U = T`, nil)
}
```

There is a typo here: `type U = T` should have been `union U = T`, but this loops infinitely instead of reporting a syntax error.