3629a87 changed the handling of cylinders in multi-dives edit.
Not only should the cylinders be the "same", but also at the
same position. The code did not check whether the edited dives
even had that many cylinders, leading to a null-pointer
dereference.
Check whether the cylinder exists before comparing it.
Fixes#3578.
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>
eb5d4f2a15