Currently, the caller is responsible for not reusing a freed
weightsystem / cylinder or resetting the description field to
null. This is very unfriendly. Set the description field to null,
because that allows us to call free_* repeatedly on the same
object. Use the new behavior to make the weightsystem model code
a bit cleaner.
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>