BUG: Fix uninitialized variables in N4BiasFieldCorrectionImageFilter.

Detected by SimpleITK's test suite.

Change-Id: Id904b4e89593b940da26349620548f3152ad3d16
Merge topic 'N4-uninitialized'

fa9ee04 BUG: Fix uninitialized variables in N4BiasFieldCorrectionImageFilter.