Out-of-Bounds Write in Sagetech MXS Message Parsing

ArduPilot's Sagetech MXS ADS-B backend did not safely handle certain receiver messages, allowing malformed or corrupted serial input to write outside the intended message buffer. This could crash the flight stack or corrupt state. The affected parser is connected to a dedicated ADS-B sensor serial link, and no remote radio-to-parser attack path was established.