fix LoadBalancerWithNaming memory leak when ssl init failed

[ehds]

What problem does this PR solve?

Issue Number:

Problem Summary:
lb (LoadBalancerWithNaming) 可能内存泄漏当 CreateSocketSSLContext 失败时。

What is changed and the side effects?

Changed:
使用 unique_ptr 包裹 lb 临时对象，并在成功完成初始化后，将其生命周期转移给 _lb.

Side effects:

• Performance effects(性能影响):
  无
• Breaking backward compatibility(向后兼容性):
  无

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Check List:

• Please make sure your changes are compilable(请确保你的更改可以通过编译).
• When providing us with a new feature, it is best to add related tests(如果你向我们增加一个新的功能, 请添加相关测试).
• Please follow Contributor Covenant Code of Conduct.(请遵循贡献者准则).

[chenBright]

LGTM