The tab was crashing if there were no cylinders because
1) per_cylinder_mean_depth() would access non-existing cylinders.
2) TabDiveInformation::updateProfile() would access a non-existing
mean.
Fix both of these crash conditions by checking whether the dive
actually has cylinders.
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>
6a1e2cd8c2