Fix bug with "Circular Structure" error

Since `-1` is truthy, every JSON.stringify error is mistaken to be `circular structure`. This commit fixes that behaviour, so that other errors like `Do not know how to serialize Bigint` (see blitz-js/babel-plugin-superjson-next#63) aren't swallowed.

packages/next/pages/_document.tsx

@@ -761,7 +761,7 @@ export class NextScript extends Component<OriginProps> {
       const data = JSON.stringify(__NEXT_DATA__)
       return htmlEscapeJsonString(data)
     } catch (err) {
-      if (err.message.indexOf('circular structure')) {
+      if (err.message.indexOf('circular structure') !== -1) {
         throw new Error(
           `Circular structure in "getInitialProps" result of page "${__NEXT_DATA__.page}". https://nextjs.org/docs/messages/circular-structure`
         )