Fix bug with merging a period into itself

When we try to merge two SegmentIndex objects, we check to make sure
that they are sequential.  Then, after merging the various SegmentIndexes,
they are fit.
This causes problems when trying to merge two identical periods together,
an operation that could potentially happen in a live stream.
SegmentIndex.merge was comparing the post-fit version of one SegmentIndex
with the pre-fit version of another, causing it to believe that the two
SegmentIndex had different end times when really they were identical.

This change causes SegmentTemplate.createStream to perform an extra fit
operation before merging, to avoid that problem.

Closes #1448

Backported to v2.4.x

Change-Id: If2e9a9a91de344b7ab8cd9004da0553b324cb599

lib/dash/segment_template.js

@@ -78,7 +78,19 @@ shaka.dash.SegmentTemplate.createStream = function(
 
     let references = SegmentTemplate.createFromTimeline_(context, info);
 
+    // Don't fit live content, since it might receive more segments.
+    // Unless that live content is multi-period; it's safe to fit every period
+    // but the last one, since only the last period might receive new segments.
+    let shouldFit = !context.dynamic || !context.periodInfo.isLastPeriod;
+
     if (segmentIndex) {
+      if (shouldFit) {
+        // Fit the new references before merging them, so that the merge
+        // algorithm has a more accurate view of their start and end times.
+        let wrapper = new shaka.media.SegmentIndex(references);
+        wrapper.fit(context.periodInfo.duration);
+      }
+
       segmentIndex.merge(references);
       let start = context.presentationTimeline.getSegmentAvailabilityStart();
       segmentIndex.evict(start - context.periodInfo.start);
@@ -91,7 +103,7 @@ shaka.dash.SegmentTemplate.createStream = function(
       }
     }
 
-    if (!context.dynamic || !context.periodInfo.isLastPeriod) {
+    if (shouldFit) {
       segmentIndex.fit(context.periodInfo.duration);
     }