SEQEND: implemented DXF_DEBUG macros.

Signed-off-by: bert <bert.timmerman@xs4all.nl>
SECTION: correction in a Doxygen comment.

Signed-off-by: bert <bert.timmerman@xs4all.nl>
SEQEND: check for NULL pointers and do basic checks.

Signed-off-by: bert <bert.timmerman@xs4all.nl>