THUMBNAIL: added dxf_thumbnail_free().

Signed-off-by: bert <bert.timmerman@xs4all.nl>
BODY: changed test for dxf version in dxf_body_read().

Reading in a body entity whilst using an illegal DXF version
is no show stopper.

Signed-off-by: bert <bert.timmerman@xs4all.nl>
BODY: correction of an index counter in dxf_body_read().

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