Fix crashes in the map and line fragments
Summary:
Fix T1373 (bug concurrent modification)
Fix T1374 (Avoid crashing when list of points is empty)
Test Plan:
Open map. See MQTT realtime positions.
App in background. Open 5 big different apps (e.g. Mindustry). Open app again. Still works.
Reviewers: valerio.bozzolan
Reviewed By: valerio.bozzolan
Subscribers: valerio.bozzolan, Libre BusTO
Tags: Libre BusTO
Differential Revision: https://gitpull.it/D212