guard against invalid yawmode values

src/modules/navigator/mission.cpp

@@ -189,8 +189,9 @@ Mission::on_active()
 	}
 
 	/* see if we need to update the current yaw heading for rotary wing types */
-	if (_navigator->get_vstatus()->is_rotary_wing 
+	if (_navigator->get_vstatus()->is_rotary_wing
 			&& _param_yawmode.get() != MISSION_YAWMODE_NONE
+			&& _param_yawmode.get() < MISSION_YAWMODE_MAX
 			&& _mission_type != MISSION_TYPE_NONE) {
 		heading_sp_update();
 	}

src/modules/navigator/mission.h

@@ -88,7 +88,8 @@ public:
 		MISSION_YAWMODE_NONE = 0,
 		MISSION_YAWMODE_FRONT_TO_WAYPOINT = 1,
 		MISSION_YAWMODE_FRONT_TO_HOME = 2,
-		MISSION_YAWMODE_BACK_TO_HOME = 3
+		MISSION_YAWMODE_BACK_TO_HOME = 3,
+		MISSION_YAWMODE_MAX = 4
 	};
 
 private: