Don't update our video speed setting before the video is loaded.

In this state, the playback rate is always 1, as we haven't had a chance
to update it yet (which currently happens in the play callback).

Fixes speed setting when jumping between YouTube videos.

inject.js

@@ -44,6 +44,9 @@ chrome.extension.sendMessage({}, function(response) {
         });
 
         target.addEventListener('ratechange', function(event) {
+          if (target.readyState === 0) {
+            return;
+          }
           var speed = this.getSpeed();
           this.speedIndicator.textContent = speed;
           tc.settings.speed = speed;