Copter: when leaving AUTO only stop mission if running

This resolves an issue in which the mission would not automatically
start from the beginning if it had previously been run to completion

ArduCopter/flight_mode.pde

@@ -210,7 +210,9 @@ static void exit_mode(uint8_t old_control_mode, uint8_t new_control_mode)
 
     // stop mission when we leave auto mode
     if (old_control_mode == AUTO) {
-        mission.stop();
+        if (mission.state() == AP_Mission::MISSION_RUNNING) {
+            mission.stop();
+        }
     }
 
     // smooth throttle transition when switching from manual to automatic flight modes