给出完整的改进代码，解决死循环的问题

为了防止在求解最优控制问题时出现死循环，可以调整 `mesh` 参数中的 `maxiteration` 和 `colpointsmax` 值，并增加一些容错处理。以下是改进后的完整代码：

% 初始参数设置
t0min = 0;
t0max = 0;
tfmin = 0;
tfmax = 30;

% 初始状态向量，包含位置和速度
h0 = [100, -200, 400, 0, 0, -10];

% 最终状态向量，包含位置和速度
hf = [0, 0, 0, 0, 0, 0];

% 状态变量的下限和上限
hmin = [-250, -250, 0, -50, -50, -50];
hmax = [450, 450, 450, 50, 50, 50];

% 控制变量的下限和上限
umin = [-3, -3, 0];
umax = [3, 3, 3];

% 重力加速度向量
setup.auxdata.g = [0, 0, 1.6];

% 边界条件设置
setup.bounds.phase.initialstate.lower = h0;
setup.bounds.phase.initialstate.upper = h0;
setup.bounds.phase.state.lower = hmin;
setup.bounds.phase.state.upper = hmax;
setup.bounds.phase.finalstate.lower = hf;
setup.bounds.phase.finalstate.upper = hf;
setup.bounds.phase.control.lower = umin;
setup.bounds.phase.control.upper = umax;
setup.bounds.phase.integral.lower = -100;
setup.bounds.phase.integral.upper = 100;
setup.bounds.phase.initialtime.lower = t0min;
setup.bounds.phase.initialtime.upper = t0max;
setup.bounds.phase.finaltime.lower = tfmin;
setup.bounds.phase.finaltime.upper = tfmax;

% 初值猜测
h_guess = [h0; hf];
u_guess = [umin; umin];
t_guess = [t0min; 5];
setup.guess.phase.state = h_guess;
setup.guess.phase.control = u_guess;
setup.guess.phase.time = t_guess;
setup.guess.phase.integral = 10;

% 设置求解器参数
setup.name = 'Moon-Lander-OCP';
setup.functions.continuous = @mlocpContinuous;
setup.functions.endpoint = @mlocpEndpoint;
setup.nlp.solver = 'snopt';
setup.derivatives.supplier = 'sparseCD';
setup.derivatives.derivativelevel = 'second';
setup.mesh.method = 'hp1';
setup.mesh.tolerance = 1e-4;
setup.mesh.maxiteration = 20; % 减少最大迭代次数
setup.mesh.colpointsmax = 4; % 减少最大节点数
setup.mesh.colpointsmin = 5; % 调整最小节点数
setup.mesh.phase.fraction = 0.1 * ones(1, 10);
setup.mesh.phase.colpoints = 4 * ones(1, 10);
setup.method = 'RPMintegration';

% 求解最优控制问题
try
    tic;
    output = gpops2(setup);
    toc;
catch ME
    fprintf('Error in solving the optimal control problem: %s\n', ME.message);
end

% 结果可视化
if exist('output', 'var') && isfield(output, 'result')
    solution = output.result.solution;
    
    figure('Color', [1, 1, 1]);
    plot(solution.phase.time, solution.phase.state(:, 1), '-x', 'LineWidth', 1.5);
    xlabel('Time', 'FontWeight', 'bold');
    ylabel('State', 'FontWeight', 'bold');
    legend('Altitude');
    set(gca, 'FontName', 'Times New Roman', 'FontSize', 15, 'LineWidth', 1.3);
    print('-dpng', 'moonlander_state_altitude.png');
    
    figure('Color', [1, 1, 1]);
    plot(solution.phase.time, solution.phase.state(:, 2), '--', 'LineWidth', 1.5);
    xlabel('Time', 'FontWeight', 'bold');
    ylabel('State', 'FontWeight', 'bold');
    legend('Velocity X');
    set(gca, 'FontName', 'Times New Roman', 'FontSize', 15, 'LineWidth', 1.3);
    print('-dpng', 'moonlander_state_velocity_x.png');
    
    figure('Color', [1, 1, 1]);
    plot(solution.phase.time, solution.phase.control, '-o', 'LineWidth', 1.5);
    xlabel('Time', 'FontWeight', 'bold');
    ylabel('Control', 'FontWeight', 'bold');
    set(gca, 'FontName', 'Times New Roman', 'FontSize', 15, 'LineWidth', 1.3);
    print('-dpng', 'moonlander_control.png');
else
    fprintf('No valid solution found.\n');
end

% 连续系统函数
function phaseout = mlocpContinuous(input)
    g = input.auxdata.g;
    t = input.phase.time;
    x = input.phase.state(:, 1:3);
    v = input.phase.state(:, 4:6);
    u = input.phase.control;
    dx = v;
    dv = u - [0, 0, 1.6];
    phaseout.dynamics = [dx, dv];
    phaseout.integrand = sum(u .^ 2, 2);
end

% 终值函数
function output = mlocpEndpoint(input)
    J = input.phase.integral;
    output.objective = sum(J);
end

### 主要改进点：
1. **减少最大迭代次数**：将 `setup.mesh.maxiteration` 从 45 减少到 20，以避免过度迭代导致的死循环。
2. **减少最大节点数**：将 `setup.mesh.colpointsmax` 从 6 减少到 4，以减少计算复杂度。
3. **增加错误捕获**：使用 `try-catch` 结构来捕获并报告求解过程中可能出现的错误。
4. **检查结果有效性**：在绘制图形之前，检查是否存在有效的解决方案。

这些改进可以帮助防止死循环并提高代码的健壮性。