fix(HLS): Avoid "Possible encoding problem detected!" when is a prelo…

…ad reference (shaka-project#5332)

Preload segments do not have a defined duration, so this log may be
wrong.

lib/media/media_source_engine.js

@@ -845,7 +845,7 @@ shaka.media.MediaSourceEngine = class {
       this.append_(contentType, data);
     });
 
-    if (goog.DEBUG && reference) {
+    if (goog.DEBUG && reference && !reference.isPreload()) {
       const bufferedAfter = this.getBuffered_(contentType);
       const newBuffered = shaka.media.TimeRangesUtils.computeAddedRange(
           bufferedBefore, bufferedAfter);