AP_Rally: Added one getter method and made a utility method public.
AP_Mission: added get_current_nav_index() function

this will return 0 when there is no current command, which is what is
expected in MAVLink when there is no mission

(it prevents the text to speech announcing "65 thousand 6 hundred and
thirty five", which is quite annoying!)
Plane: use get_current_nav_index()
Rover: use get_current_nav_index()