client端SSL相关问题。

**Describe the bug (描述bug)**
最近某些服务在使用https过程中发现ssl有时会出现异常，具体表现在 socket.cpp中的DoRead函数（write也会有类似异常）。打印出 Fail to read from ssl_fd=id：Success

代码：

```
ssize_t nr = _read_buf.append_from_SSL_channel(_ssl_session, &ssl_error, size_hint);
switch (ssl_error) {
case SSL_ERROR_NONE: // `nr' > 0
break;

case SSL_ERROR_WANT_READ:
// Regard this error as EAGAIN
errno = EAGAIN;
break;

case SSL_ERROR_WANT_WRITE:
// Disable renegotiation
errno = EPROTO;
return -1;

default: {
const unsigned long e = ERR_get_error();
if (nr == 0) {
// Socket EOF or SSL session EOF
} else if (e != 0) {
LOG(WARNING) << "Fail to read from ssl_fd=" << fd()
<< ": " << SSLError(e);
errno = ESSL;
} else {
// System error with corresponding errno set
PLOG(WARNING) << "Fail to read from ssl_fd=" << fd();
}
break;
}
}
return nr;
```
因为未打印出错误码，无法定位Success是何种错误，此外，代码中似乎认为nr >0 一定对应了SSL_ERROR_NONE。不知道这种判断的依据是？发现ssl出现异常后，nr>0, 且errno = ESSL（），出现这种情况的原因暂时不确定，只能猜测是ssl context出现异常，且该异常出现后，后续ssl皆无法正常工作。因此想知道对这种异常应该如何处理？目前看Read和Write阶段均未判断这种情况，且都出现了类似的异常。
**To Reproduce (复现方法)**
未知。

Start with socket.cpp's DoRead function and the analogous write path, tracing _read_buf.append_from_SSL_channel's nr, ssl_error, errno, and ERR_get_error handling. No reproduction method or test is provided, so first establish the SSL failure; done means the behavior of nr > 0 with an SSL error is understood and verified with a regression test.

Written by the indexing model from the issue text.