Fix Coverity #76266: Initialize members, make constructor protected

FieldInterface is a purely abstract class used as a base class for
KBlocksField. Thus its constructor should be protected meaning that it
can be called during the construction of KBlocksField, but calling
FieldInterface fi = new FieldInterface() would fail.

Also, the member variables are now initialized to their default values
maBoard = nullptr, mWidth = 0, and mHeight = 0.