Bug 1319992: P5. Don't attempt to estimate readyState when ended. r=jwwang

Buffered range is now calculated asynchronously. It may not be up to date by the time the MDSM has entered ended mode.
There's no point estimating readyState in ended mode anyway: we know what it is...

MozReview-Commit-ID: ErGsAwBzeXI

dom/html/HTMLMediaElement.cpp

@@ -5474,7 +5474,7 @@ HTMLMediaElement::UpdateReadyStateInternal()
       // 6. Suspend playback.
       // Note: Playback will already be stalled, as the next frame is
       // unavailable.
-    } else if (mDecoder) {
+    } else if (mDecoder && !mDecoder->IsEnded()) {
       nextFrameStatus = mDecoder->NextFrameBufferedStatus();
     }
   }