Waterfall plotting: Unhandled IndexError when trying to read invalid waterfall file

The following error message was created on station 132 during observation 1374809:

IndexError: index 0 is out of bounds for axis 0 with size 0
  File "apscheduler/executors/base.py", line 125, in run_job
    retval = job.func(*job.args, **job.kwargs)
  File "satnogsclient/scheduler/tasks.py", line 86, in spawn_observer
    observer.observe()
  File "satnogsclient/observer/observer.py", line 161, in observe
    self.plot_waterfall()
  File "satnogsclient/observer/observer.py", line 254, in plot_waterfall
    waterfall_png=self.observation_waterfall_png)
  File "satnogsclient/observer/waterfall.py", line 18, in plot_waterfall
    nchan = int(np.fromfile(wf_file, dtype='float32', count=1)[0])

This might be caused by non-available disk space, as this station stopped the transmission of waterfalls some hours ago, possibly caused by #318 (closed).