Fix race condition on file SD (#3468)
Fix race condition on file SD (#3468)

The file discovery should only stop the watcher if it has been created
otherwise it may trigger a segmentation fault.
GoogleCloudPlatform/prometheus,Conorbro/prometheus,AlekSi/prometheus,Stackdriver/stackdriver-prometheus,Stackdriver/stackdriver-prometheus,keegancsmith/prometheus,Conorbro/prometheus,keegancsmith/prometheus,amorken/prometheus,AlekSi/prometheus,kangwoo/prometheus,prometheus/prometheus,alileza/prometheus,amorken/prometheus,alileza/prometheus,prometheus/prometheus,brancz/prometheus,Conorbro/prometheus,keegancsmith/prometheus,Gouthamve/prometheus,AlekSi/prometheus,kangwoo/prometheus,GoogleCloudPlatform/prometheus,prometheus/prometheus,Conorbro/prometheus,GoogleCloudPlatform/prometheus,brancz/prometheus,Gouthamve/prometheus,Gouthamve/prometheus,Stackdriver/stackdriver-prometheus,GoogleCloudPlatform/prometheus,amorken/prometheus,keegancsmith/prometheus,alileza/prometheus,GoogleCloudPlatform/prometheus,keegancsmith/prometheus,alileza/prometheus,Gouthamve/prometheus,prometheus/prometheus,kangwoo/prometheus,AlekSi/prometheus,prometheus/prometheus,amorken/prometheus,kangwoo/prometheus,brancz/prometheus,alileza/prometheus,Stackdriver/stackdriver-prometheus,brancz/prometheus