VideoTexture: Make update() more robust

src/textures/VideoTexture.js

@@ -14,6 +14,8 @@ function VideoTexture( video, mapping, wrapS, wrapT, magFilter, minFilter, forma
 
 	function update() {
 
+		var video = scope.image;
+
 		if ( video.readyState >= video.HAVE_CURRENT_DATA ) {
 
 			scope.needsUpdate = true;