Silence a warning about missing return

Some versions of the compiler do not seem to recognize this as
unreachable, so add a return to satisfy the compiler and silence
the warning.

Change-Id: I55485d2847616be696b6c4f908e2042cb4c00392

lib/dash/mpd_processor.js

@@ -1064,6 +1064,7 @@ shaka.dash.MpdProcessor.prototype.makeSegmentIndexSourceViaSegmentTemplate_ =
   }
 
   shaka.asserts.unreachable();
+  return null;
 };