fix the bug that the first caption and the second one will become current at the…

fix the bug that the first caption and the second one will become current at the same time when displaying right captions

common/lib/xmodule/xmodule/js/src/video/09_video_caption.js

@@ -453,6 +453,9 @@ function () {
             min = 0;
             max = this.videoCaption.start.length - 1;
 
+            if (time < this.videoCaption.start[min]) {
+                return -1;
+            }
             while (min < max) {
                 index = Math.ceil((max + min) / 2);
 
@@ -507,10 +510,11 @@ function () {
             newIndex = this.videoCaption.search(time);
 
             if (
-                newIndex !== void 0 &&
+                typeof newIndex !== 'undefined' &&
+                newIndex !== -1 &&
                 this.videoCaption.currentIndex !== newIndex
             ) {
-                if (this.videoCaption.currentIndex) {
+                if (typeof this.videoCaption.currentIndex !== 'undefined') {
                     this.videoCaption.subtitlesEl
                         .find('li.current')
                         .removeClass('current');