check to make sure keep_temporary_files is defined

Author: rwblair

nipype/interfaces/ants/segmentation.py

@@ -701,7 +701,7 @@ def _list_outputs(self):
                                                        self.inputs.out_prefix +
                                                        'BrainExtractionBrain.' +
                                                        self.inputs.image_suffix)
-        if self.inputs.keep_temporary_files != 0:
+        if isdefined(self.inputs.keep_temporary_files) && self.inputs.keep_temporary_files != 0:
             outputs['BrainExtractionCSF'] = os.path.join(
                 os.getcwd(),
                 self.inputs.out_prefix + 'BrainExtractionCSF.' + self.inputs.image_suffix