Fix AppRTCDemo crash when room is connected after PC is destroyed.

Also move VideoRendererGui.dispose() to the section with public API.

BUG=4909
R=wzh@webrtc.org

Review URL: https://codereview.webrtc.org/1312523004 .

Cr-Commit-Position: refs/heads/master@{#9792}

webrtc/examples/androidapp/src/org/appspot/apprtc/CallActivity.java

@@ -369,7 +369,10 @@ public class CallActivity extends Activity
   private void callConnected() {
     final long delta = System.currentTimeMillis() - callStartedTimeMs;
     Log.i(TAG, "Call connected: delay=" + delta + "ms");
-
+    if (peerConnectionClient == null || isError) {
+      Log.w(TAG, "Call is connected in closed or error state");
+      return;
+    }
     // Update video view.
     updateVideoView();
     // Enable statistics callback.