PX4 autopilot has a heap Use-After-Free in MavlinkShell::available() via SERIAL_CONTROL Race Condition

PX4 autopilot is a flight control solution for drones. Prior to 1.17.0-rc1, a heap-use-after-free is detected in the MavlinkShell::available() function. The issue is caused by a race condition between the MAVLink receiver thread (which handles shell creation/destruction) and the telemetry sender thread (which polls the shell for available output). The issue is remotely triggerable via MAVLink SERIAL_CONTROL messages (ID 126), which can be sent by an external ground station or automated script. This vulnerability is fixed in 1.17.0-rc1.