fix: resolve memory leak by adding proper event listener and timer cl…

[sangjiexun]

…eanup

• player.js: Store bound handlers and remove all event listeners in destroy()
• base_client.js: Call stopStreamFlush() in destroy() to clear timers
• mse.js: Remove sourceopen/sourceended/sourceclose listeners in destroy()
• MSEBuffer: Add null check for sourceBuffer before removal
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• player.js：存储绑定的处理函数，并在 destroy() 方法中移除所有事件监听器
• base_client.js：在 destroy() 中调用 stopStreamFlush() 以清除定时器
• mse.js：在 destroy() 中移除 sourceopen、sourceended、sourceclose 事件监听器
• MSEBuffer：在移除前对 sourceBuffer 增加空值判断