Corrected Freesurfer SegStats

Corrected Freesurfer SegStats _list_outputs to avoid error if
summary_file is undefined (issue #994)

Author: blakedewey

nipype/interfaces/freesurfer/model.py

@@ -635,8 +635,9 @@ class SegStats(FSCommand):
 
     def _list_outputs(self):
         outputs = self.output_spec().get()
-        outputs['summary_file'] = os.path.abspath(self.inputs.summary_file)
-        if not isdefined(outputs['summary_file']):
+        if isdefined(self.inputs.summary_file):
+        	outputs['summary_file'] = os.path.abspath(self.inputs.summary_file)
+        else:
             outputs['summary_file'] = os.path.join(os.getcwd(), 'summary.stats')
         suffices = dict(avgwf_txt_file='_avgwf.txt', avgwf_file='_avgwf.nii.gz',
                         sf_avg_file='sfavg.txt')