Added comments from the Directives Wiki into the default configuration file, and fixed a segfault when starting SLS with no config file

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meant to be 2 separate PRs for your consideration, but forgot to create a branch :(

• Added the comments from the directives Wiki to the sls.conf
• When SLS was started without a "-c", there was a segfault at the end:

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           srt-live-srver 
                    v1.4.x 
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2020-09-19 13:58:56:239 SLS INFO: sls_conf_open, parsing conf file='./sls.conf'.
2020-09-19 13:58:56:239 SLS FATAL: open conf file='./sls.conf' failed, please check if the file exist.
2020-09-19 13:58:56:239 SLS INFO: sls_conf_open failed, EXIT!
2020-09-19 13:58:56:239 SLS INFO: exit, stop srt live server...
2020-09-19 13:58:56:239 SLS INFO: exit, release reload_manager_list begin，count=0.
2020-09-19 13:58:56:239 SLS INFO: exit, release reload_manager_list ok.
2020-09-19 13:58:56:239 SLS INFO: exit, release http_stat_client.
2020-09-19 13:58:56:239 SLS INFO: exit, uninit srt .
2020-09-19 13:58:56:239 SLS INFO: exit, close conf.
Segmentation fault

Just checking c != null on closing the config fixes the issue.

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blew it again :( I integrated @odensc pull request to fix stats and didn't realise that it would be included here.

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closing as my branch is messed out and the PR was very simple !