Guard against no iframe contentWindow in Vimeo player

Partially fixes cookpete/react-player#2

src/players/Vimeo.js

@@ -68,7 +68,7 @@ export default class Vimeo extends Base {
   postMessage = (method, value) => {
     if (!this.origin) return
     let data = JSON.stringify({ method, value })
-    return this.iframe.contentWindow.postMessage(data, this.origin)
+    return this.iframe.contentWindow && this.iframe.contentWindow.postMessage(data, this.origin)
   }
   render () {
     let style = {