NifsMerge crashes when only a single cube to merge is passed in.

[Nat1405]

Relevant log:

==> GN-2015A-Q-6.err <==
Traceback (most recent call last):
  File "/opt/conda/envs/iraf27/bin/runNifty", line 38, in <module>
    nifty.nifsPipeline.start(sys.argv[2:])
  File "/opt/conda/envs/iraf27/lib/python2.7/site-packages/nifty/pipeline/nifsPipeline.py", line 223, in start
    nifsMerge.run()
  File "/opt/conda/envs/iraf27/lib/python2.7/site-packages/nifty/pipeline/steps/nifsMerge.py", line 102, in run
    mergeCubes(scienceDirectoryList, "uncorrected", mergeType, use_pq_offsets, im3dtran, over)
  File "/opt/conda/envs/iraf27/lib/python2.7/site-packages/nifty/pipeline/steps/nifsMerge.py", line 412, in mergeCubes
    iraf.imcombine(prefix+'*', output = 'cube_merged.fits',  combine = mergeType, offsets = 'offsets.txt')
UnboundLocalError: local variable 'prefix' referenced before assignment