fix: exclude "future" segments from presentation timeline calculations

lib/media/presentation_timeline.js

@@ -236,7 +236,16 @@ shaka.media.PresentationTimeline = class {
 
     let firstReferenceStartTime = references[0].startTime;
     let lastReferenceEndTime = references[0].endTime;
+
+    // Date.now() is in milliseconds, from which we compute "now" in seconds.
+    const now = (Date.now() + this.clockOffset_) / 1000.0;
+
     for (const reference of references) {
+      // Stop if we've reached segments that are in the "future".
+      if (now < reference.startTime) {
+        break;
+      }
+
       firstReferenceStartTime = Math.min(
           firstReferenceStartTime, reference.startTime);
       lastReferenceEndTime = Math.max(lastReferenceEndTime, reference.endTime);
@@ -251,8 +260,6 @@ shaka.media.PresentationTimeline = class {
         !this.startTimeLocked_) {
       // Since we have explicit segment end times, calculate a presentation
       // start based on them.  This start time accounts for drift.
-      // Date.now() is in milliseconds, from which we compute "now" in seconds.
-      const now = (Date.now() + this.clockOffset_) / 1000.0;
       this.presentationStartTime_ =
           now - this.maxSegmentEndTime_ - this.maxSegmentDuration_;
     }