fix IndexError raised from load_data

Observed traceback:
Traceback (most recent call last):
 File "/usr/lib/nagios/plugins/check_iftraffic_nrpe.py", line 527, in <module>
   main(default_values)
 File "/usr/lib/nagios/plugins/check_iftraffic_nrpe.py", line 376, in main
   uptime0, time0, if_data0 = load_data(args.data_file, _counters)
 File "/usr/lib/nagios/plugins/check_iftraffic_nrpe.py", line 131, in load_data
   device_name = data.pop(0)
IndexError: pop from empty list

Resolution: Assume that the data file is truncated, recreate it and
return UNKNOWN to Nagios.