Fix - Handle when outside message event.data is not a string

[WayneEllery]

Fixed issue when outside message data was not a string.

Steps to reproduce:

1. Load page with safeframe
2. Open web developer tools console
3. Type window.postMessage({}, "*") and enter

Result:

Uncaught TypeError: data.indexOf is not a function
    at _handle_msg_from_outside (host.js:1729)

Expected result:

No error and message should be ignored