getIndexBasedSegment() can return null but there is no prevent code

Created by: uyeong

Environment

• The MPD passes the DASH-IF Conformance Tool on https://conformance.dashif.org/
• The stream has correct Access-Control-Allow-Origin headers (CORS)
• There are no network errors such as 404s in the browser console when trying to play the stream
• The issue observed is not mentioned on https://github.com/Dash-Industry-Forum/dash.js/wiki/FAQ
• The issue occurs in the latest reference client on http://reference.dashif.org/dash.js/ and not just on my page

• Link to playable MPD file:
• Dash.js version: 3.1.0
• Browser name/version: All
• OS name/version: All

Depending on the packaging state, getIndexBasedSegment() sometimes returns null. but there is no prevent code.

src/dash/utils/ListSegmentsGetter.js

getIndexBasedSegment() that is called by getSegmentByIndex() may return null, so i think it need code to prevent null reference error.