Tailsitter VTOLs very occasionally gets stuck with zero roll and pitch angle in multicopter mode after
a forward transition command is issued.
This very rare behavior is triggered by the following events:
1> a forward transition is triggered either in auto or manual mode.
2> in the vtol_att_control main loop, if multicopter and fixed wing attitude setpoints are not updated, transition state is not updated
3> the commander changes the vehicle status to transition mode.
4> multicopter pos controller initiated Transition flight task. This results in zero roll and pitch setpoint due to zero acceleration setpoint
5> now vtol_att_control executes and updates the transition state. Specifically, _q_trans_start and _q_trans_sp are set with zero roll and pitch sp
6> tilt is evaluated to be NaN, despite _q_trans_sp being normalized. This happens for 25% of all yaw angles when using float datatype. This can be
verified using the matrix library
7> once tilt is evaluated to be NaN, _q_trans_sp is never updated again and is stuck in this state for ever.
This has been fixed by constraining the cos(tilt) within +1 to -1 range
Further, _q_trans_start and _q_trans_sp are immedietly initialized after a transition event is triggered.
