Fix demo app when no text tracks.

If there were no text tracks, the demo app would read out of bounds.
This didn't cause any visible changes only an error log.

Change-Id: If6f37d26f1236a8e55606cd45e3d1d9a34783d6b

Author: TheModMaker

demo/info_section.js

@@ -57,7 +57,9 @@ shakaDemo.onTracksChanged_ = function(event) {
 shakaDemo.updateVariantTracks_ = function() {
   var trackList = document.getElementById('variantTracks');
   var langList = document.getElementById('audioLanguages');
-  var language = langList.options[langList.selectedIndex].value;
+  var language = langList.selectedIndex >= 0 ?
+      langList.options[langList.selectedIndex].value :
+      '';
 
   var tracks = shakaDemo.player_.getVariantTracks();
 
@@ -77,7 +79,9 @@ shakaDemo.updateTextTracks_ = function() {
   var trackList = document.getElementById('textTracks');
 
   var langList = document.getElementById('textLanguages');
-  var language = langList.options[langList.selectedIndex].value;
+  var language = langList.selectedIndex >= 0 ?
+      langList.options[langList.selectedIndex].value :
+      '';
 
   var tracks = shakaDemo.player_.getTextTracks();