Fixes another null exception in cast receiver

When calling getStats, if there is no active stream but there is
a playhead and manifest, the player experiences a null exception.
This does not fully fix #1128, however, as there seems to be more than
a null exception at work there at the moment.

Issue #1128

Change-Id: Ic9d318a037157c2a8e28a54a857ec3a1e5797aae

lib/player.js

@@ -1527,12 +1527,14 @@ shaka.Player.prototype.getStats = function() {
     var period = this.manifest_.periods[periodIdx];
     var activeStreams = this.activeStreamsByPeriod_[periodIdx];
 
-    variant = shaka.util.StreamUtils.getVariantByStreamIds(
-        activeStreams[ContentType.AUDIO],
-        activeStreams[ContentType.VIDEO],
-        period.variants);
+    if (activeStreams) {
+      variant = shaka.util.StreamUtils.getVariantByStreamIds(
+          activeStreams[ContentType.AUDIO],
+          activeStreams[ContentType.VIDEO],
+          period.variants);
 
-    video = variant.video || {};
+      video = variant.video || {};
+    }
   }
 
   if (!video) video = {};