Fix random value of CachedReader at first time

[chenBright]

What problem does this PR solve?

Issue Number:

Problem Summary:

CachedReader::get_value中fn没有在锁内执行，第一次执行get_value的时候，可能会读到_cache的初始值。由于_cache没有显式初始化，而且实际的类型（如ProcMemory、ProcStat）没有实现默认构造函数，所以成员变量的值是随机值。相对于随机值，值为0，让人更好理解一些。

brpc/src/bvar/default_variables.cpp

Lines 147 to 163 in 24fc31e

	pthread_mutex_lock(&p->_mutex);
	if (now > p->_mtime_us + CACHED_INTERVAL_US) {
	p->_mtime_us = now;
	pthread_mutex_unlock(&p->_mutex);
	// don't run fn inside lock otherwise a slow fn may
	// block all concurrent bvar dumppers. (e.g. /vars)
	T result;
	if (fn(&result)) {
	pthread_mutex_lock(&p->_mutex);
	p->_cached = result;
	} else {
	pthread_mutex_lock(&p->_mutex);
	}
	}
	pthread_mutex_unlock(&p->_mutex);
	}
	return p->_cached;

What is changed and the side effects?

Changed:

Side effects:

• Performance effects(性能影响):

• Breaking backward compatibility(向后兼容性):

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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).

[wwbmmm]

LGTM

[serverglen]

LGTM