Flight tasks: avoid using *= for scalar to vector assignment

It's confusing and there is a corner case where the result is incorrect:
*= 0 will not set the variable to 0 if it's already NAN.

src/lib/FlightTasks/tasks/Auto/FlightTaskAuto.cpp

@@ -136,7 +136,7 @@ bool FlightTaskAuto::_evaluateTriplets()
 		} else {
 			tmp_target(0) = _lock_position_xy(0);
 			tmp_target(1) = _lock_position_xy(1);
-			_lock_position_xy *= NAN;
+			_lock_position_xy.setAll(NAN);
 		}
 
 	} else {

src/lib/FlightTasks/tasks/ManualPositionSmooth/FlightTaskManualPositionSmooth.cpp

@@ -57,7 +57,7 @@ void FlightTaskManualPositionSmooth::_updateSetpoints()
 	_velocity_setpoint(0) = vel_sp_xy(0);
 	_velocity_setpoint(1) = vel_sp_xy(1);
 
-	/* Check for altitude lock.*/
+	/* Check for xy position lock.*/
 	_updateXYlock();
 
 	/* Smooth velocity in z.*/

src/lib/FlightTasks/tasks/Offboard/FlightTaskOffboard.cpp

@@ -68,8 +68,8 @@ bool FlightTaskOffboard::activate()
 {
 	bool ret = FlightTask::activate();
 	_position_setpoint = _position;
-	_velocity_setpoint *= 0.0f;
-	_position_lock *= NAN;
+	_velocity_setpoint.setZero();
+	_position_lock.setAll(NAN);
 	return ret;
 }
 
@@ -117,7 +117,7 @@ bool FlightTaskOffboard::update()
 		return true;
 
 	} else {
-		_position_lock *= NAN;
+		_position_lock.setAll(NAN);
 	}
 
 	// Takeoff
@@ -135,7 +135,7 @@ bool FlightTaskOffboard::update()
 		return true;
 
 	} else {
-		_position_lock *= NAN;
+		_position_lock.setAll(NAN);
 	}
 
 	// Land
@@ -155,7 +155,7 @@ bool FlightTaskOffboard::update()
 		return true;
 
 	} else {
-		_position_lock *= NAN;
+		_position_lock.setAll(NAN);
 	}
 
 	// IDLE