ncjoin.py fails when second input file is short

[jotelha]

When concatenating NetCDF trajectories, ncjoin.py may fail with an error like this,

Traceback (most recent call last):
  File "/p/project/hfr21/common/juwels/easybuild/stages/2019a/software/imteksimcs/devel-intel-2019a-Python-3.6.8/bin/ncjoin", line 10, in <module>
    sys.exit(main())
  File "/p/project/hfr21/common/juwels/easybuild/stages/2019a/software/imteksimcs/devel-intel-2019a-Python-3.6.8/lib/python3.6/site-packages/imteksimcs/cli/NetCDF/ncjoin.py", line 111, in main
    test_tol=arguments.test_tol)
  File "/p/project/hfr21/common/juwels/easybuild/stages/2019a/software/imteksimcs/devel-intel-2019a-Python-3.6.8/lib/python3.6/site-packages/imteksimcs/NetCDF/ncjoin.py", line 110, in open_trajs
    test2, dummy = fix_time(test2)
  File "/p/project/hfr21/common/juwels/easybuild/stages/2019a/software/imteksimcs/devel-intel-2019a-Python-3.6.8/lib/python3.6/site-packages/imteksimcs/NetCDF/ncjoin.py", line 63, in fix_time
    if len(time) > 1 and abs(dtime-(time[1]-time[0])) > 1e-3:
TypeError: unsupported operand type(s) for -: 'NoneType' and 'float'

for a call like this

'['ncjoin', '-v', 'time', '-f', 'traj.nc', '-x', '--', 'previous.default.nc', 'default.nc']'

and my suspicion here is that this happens when the second input nc file has only one frame.