weird bug in new psf

[ryanvarley]

Traceback (most recent call last):3 done (samp 437/447)
  File "run_visit.py", line 179, in <module>
    obs.run_observation()
  File "/Users/ryan/PythonPackages/WFC3Sim/observation.py", line 344, in run_observation
    self._generate_exposure(start_time, filenum)
  File "/Users/ryan/PythonPackages/WFC3Sim/observation.py", line 406, in _generate_exposure
    progress_bar=self.progess,
  File "/Users/ryan/PythonPackages/WFC3Sim/exposure_generator.py", line 316, in scanning_frame
    scale_factor, add_flat)
  File "/Users/ryan/PythonPackages/WFC3Sim/exposure_generator.py", line 642, in _gen_subsample
    wl_psf_std, pixel_array)
  File "/Users/ryan/PythonPackages/WFC3Sim/exposure_generator.py", line 750, in _psf_distribution
    yx = np.insert(yx, len(yx), np.zeros(yx_size - len(yx)))
ValueError: negative dimensions are not allowed

only happens sometimes, but repeatably to the same part and seems to depend on seed.