ModemManager: ModemManager: Reachable assertion in Cell Broadcast encoding parse causes denial of service

A flaw was found in ModemManager. When parsing a Cell Broadcast Message, some 3GPP data-coding-scheme values (8-bit and reserved character sets) are not handled. The process hits a reachable assertion and aborts. An attacker who can deliver a crafted Cell Broadcast PDU over the radio network, or via a modem AT channel, can cause ModemManager to exit. Repeated aborts can exhaust systemd's default start limit and leave the service failed.