Fix occasional crashes in python plugins

Occasionally python plugins such as the nginx plugin crash with

```
Exception in thread local:
Traceback (most recent call last):
  File "/usr/lib64/python3.4/threading.py", line 911, in _bootstrap_inner
    self.run()
  File "/usr/libexec/netdata/python.d/python_modules/base.py", line 166, in run
    time.sleep(self.timetable['next'] - time.time())
ValueError: sleep length must be non-negative
```

Afterwards no further data is collected for the plugin until netdata
is restarted