avmerge: Fix errors if close is called twice

This can happen if the quality is changed from an avmerge source
to an integrated source, and then changed back to an avmerge source

Close is called the first change, but on the last change, it's
called again because the avMerge object is still present, resulting
in InvalidStateError

Signed-off-by: Jesús <heckyel@hyperbola.info>

youtube/static/js/av-merge.js

@@ -37,6 +37,7 @@ function AVMerge(video, srcPair, startTime){
     this.startTime = startTime;
     this.video = video;
     this.mediaSource = null;
+    this.closed = false;
     this.setup();
 }
 AVMerge.prototype.setup = function() {
@@ -67,6 +68,9 @@ AVMerge.prototype.sourceOpen = function(_) {
     //this.video.onseeked = function() {console.log('seeked')};
 }
 AVMerge.prototype.close = function() {
+    if (this.closed)
+        return;
+    this.closed = true;
     this.videoStream.close();
     this.audioStream.close();
     this.timeUpdateEvt.remove();