Remove unused fields, ctor-initialize others.

Fixes Coverity 1091336, 1091337, 1091338
Fix Coverity 1091331 by avoiding a null dereference.

This is actually a false positive because m_subScreen is only NULL
when using mythccextractor which would never call the method
indicated.
Fix Coverity 700919.  Initialize a field in default ctor.

This may have been a real issue (though I couldn't get anything bad to
happen in testing).