fix: Fix HLS lazy-loading exception during update

If a stream switch occurs during a playlist update, that update may
need to be cancelled.  This handles this edge case to prevent an
exception trying to access stream.segmentIndex.

Closes #4647

lib/hls/hls_parser.js

@@ -347,6 +347,10 @@ shaka.hls.HlsParser = class {
       uriObj.setQueryData(new goog.Uri.QueryData('_HLS_skip=YES'));
     }
     const response = await this.requestManifest_(uriObj.toString());
+    if (!streamInfo.stream.segmentIndex) {
+      // The stream was closed since the update was first requested.
+      return;
+    }
 
     /** @type {shaka.hls.Playlist} */
     const playlist = this.manifestTextParser_.parsePlaylist(