Fixed the error bad_module when using a custom log path.

If a log path is not `"log/ejabberd.log"` (for example, `EJABBERD_LOG_PATH` is set), then the error will occur.

```
{"init terminating in do_boot",
 {{badmatch,{error,{bad_return,{{ejabberd_app,start,
                                              [normal,[]]},
                                {'EXIT',{{badmatch,{error,bad_module}},
                                         [{ejabberd_loglevel,set,1,
                                                             [{file,"src/ejabberd_loglevel.erl"},{line,62}]},
                                          {ejabberd_app,start,2,
                                                        [{file,"src/ejabberd_app.erl"},{line,43}]},
                                          {application_master,start_it_old,4,
                                                              [{file,"application_master.erl"},{line,274}]}]}}}}}},
  [{ejabberd,start,0,[{file,"src/ejabberd.erl"},{line,36}]},
   {init,start_it,1,[]},
   {init,start_em,1,[]}]}}
```