FIX: Coverity defects

git-svn-id: https://www.nitrc.org/svn/cmtk/trunk@4888 42a5c34f-2066-0410-bec5-ba365beb4995
FIX: prevent creation of header with unset data type

git-svn-id: https://www.nitrc.org/svn/cmtk/trunk@4889 42a5c34f-2066-0410-bec5-ba365beb4995