Fixes a bug reported in
https://groups.google.com/g/subsurface-divelog/c/8N3cTz2Zv5E:
When planning a CCR dive with OC bailout, the diluent gas may be chosen
as the first OC bailout gas, despite being set up with a use type of
'diluent', and likely not being available for open circuit breathing.
`best_first_ascend_cylinder` is now initialised to an invalid value
(instead of the first cylinder, which may or may not be a diluent
cylinder), and its subsequent use is guarded by a validity check.
Signed-off-by: Michael Keller <github@ike.ch>
ec0c6833a0