fix: clusterBasic default value bug

[liuzheng]

配置文件bug，文档上说免配置部分参数，实际上代码bug，未做到

bug复现

参考文档：https://github.com/baidu/bfe/blob/develop/docs/zh_cn/example/route.md

其中Step 3. 配置集群的基础信息 (conf/server_data_conf/cluster_conf.data) 配置集群cluster_demo_static和cluster_demo_dynamic健康检查的参数，其他均使用默认值

按照文档配置，报错

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x715ac5]

goroutine 1 [running]:
github.com/baidu/bfe/bfe_route/bfe_cluster.(*BfeCluster).BasicInit(0xc00009e500, 0x0, 0xc00033d700, 0x0, 0x0)
        /bfe/bfe_route/bfe_cluster/bfe_cluster.go:65 +0x55
github.com/baidu/bfe/bfe_route.(*ClusterTable).BasicInit(0xc000390220, 0xc000341ec0, 0xc000096490)
        /bfe/bfe_route/cluster_table.go:71 +0x28c
github.com/baidu/bfe/bfe_route.(*ClusterTable).Init(0xc000390220, 0xc0000b5740, 0x27, 0x0, 0xc00038c540)
        /bfe/bfe_route/cluster_table.go:55 +0x9e
github.com/baidu/bfe/bfe_route.(*ServerDataConf).clusterTableLoad(0xc000341640, 0xc0000b5740, 0x27, 0xc0000b5590, 0x23)
        /bfe/bfe_route/server_data_conf.go:100 +0x47

[CLAassistant]

All committers have signed the CLA.

[codecov-io]

Codecov Report

Merging #386 into master will decrease coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #386      +/-   ##
==========================================
- Coverage   53.01%   52.96%   -0.05%     
==========================================
  Files         244      244              
  Lines       22971    22971              
==========================================
- Hits        12177    12167      -10     
- Misses       9393     9400       +7     
- Partials     1401     1404       +3     

Impacted Files	Coverage Δ
...bfe_cluster_conf/cluster_conf/cluster_conf_load.go	51.25% <100.00%> (ø)
bfe_util/fixed_pool.go	83.33% <0.00%> (-16.67%)	⬇️
bfe_spdy/server_write_sched.go	78.46% <0.00%> (-1.54%)	⬇️
bfe_websocket/server_conn.go	76.51% <0.00%> (-1.52%)	⬇️
bfe_spdy/server_conn.go	78.09% <0.00%> (-0.89%)	⬇️
bfe_http2/server.go	71.83% <0.00%> (ø)
bfe_spdy/frame_write.go	50.50% <0.00%> (ø)

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[liuzheng]

表示是在v0.8.0的分支上做的测试，刚发现 #382

有点郁闷，在思考

[iyangsj]

@liuzheng 感谢你的关注和贡献！大家也发现这个问题了

下次可以写代码前，可以提前告知登记一个bug issue ，认领后再启动开发

[iyangsj]

@liuzheng 请将微信号发送至 bfe-osc@baidu.com（标题：申请加入开源BFE开发者社区）
我们会邀请您加入开源BFE开发者微信群方便交流

[liuzheng]

@iyangsj 遇到bug了顺手修了😊