BUG: Fix uninitialized pointer fields

Fixes Coverity issues:
1081502
1081431
1081516
1081519
1081561
1103579
1103587
1103591
1103599
1103600
1103605
1103606
1103607
1103608
1103610
1103611
1103614

Change-Id: If114decc27c8b03c33808e91e346b032e35dae08
Merge topic 'cov_uninit_pointer'

534da39b BUG: Fix uninitialized pointer fields
COMP: ImageRegistrationHistogramPlotter unchecked dynamic_cast.

From Coverity:

  CID 1175336 (#1 of 1): Unchecked dynamic_cast (FORWARD_NULL)
  4. var_deref_model: Passing null pointer "optimizer" to function
     "itk::RegularStepGradientDescentBaseOptimizer::GetCurrentIteration()
     const", which dereferences it. (The dereference happens because this is a
     virtual function call.)

Change-Id: Ie49596c0db7b413f37cdce4bb2332360f535588e
Merge topic 'cov_uninit_pointer'

1c946654 COMP: ImageRegistrationHistogramPlotter unchecked dynamic_cast.