planex-make-srpm does not handle missing arguments gracefully

[euanh]

[root@ec799f010644 /]# planex-make-srpm 
Traceback (most recent call last):
  File "/usr/bin/planex-make-srpm", line 9, in <module>
    load_entry_point('planex==0.7.3-1-s0-0.7.3-16-g7150042.el7.centos', 'console_scripts', 'planex-make-srpm')()
  File "/usr/lib/python2.7/site-packages/planex/makesrpm.py", line 170, in _main
    main(sys.argv[1:])
  File "/usr/lib/python2.7/site-packages/planex/makesrpm.py", line 129, in main
    target = os.path.splitext(os.path.basename(passthrough_args[0]))[0]
IndexError: list index out of range