Cast name to String to ensure .match is available

Summary:
PR for issue #12444

In `validateProjectName` in `react-native-cli` `name.match` is called but it's not certain that `name`is a String. This casts `name`to a String before doing the match.

I didn't find any tests for this file so I didn't add any.

Reproduce:
```
cd react-native-cli
npm install
node index.js init 123
```

Before this PR the code throws an exception, after this PR the intended `console.error` is printed.
Closes https://github.com/facebook/react-native/pull/12447

Differential Revision: D4584041

fbshipit-source-id: 117e76570aa9975ac851192b030c5a77daf19bcc