Fix uninitialized variable warning in vtkLookupTableWithEnabling

This fixes a uninitialized variable warning for the
vtkLookupTableWithEnabling class by always initializing the
logRange variable.

Change-Id: I72f07bcb450a997677e005ceb84cb5a268b2257d
Merge topic 'fix-uninitialized-variable-warning' into master

2086d001 Fix uninitialized variable warning in vtkLookupTableWithEnabling