r->header_in.user_agent可能为NULL

ngx_http_user_agent_module.c的427行

``` c
user_agent = &(r->headers_in.user_agent->value);
```

这里字段值可能为NULL

Tengine里面的这个模块判断了这个字段.....

Start at ngx_http_user_agent_module.c line 427 and inspect how r->headers_in.user_agent is used there. Compare the corresponding Tengine module logic mentioned in the issue, then verify that requests without this header no longer cause a NULL-related failure while normal user-agent matching remains intact.

Written by the indexing model from the issue text.