Fix the OSTC3 missing initial gas mix detection.
Fix the OSTC3 missing initial gas mix detection.

For the OSTC3 compatible devices, a missing initial gas mix (e.g. no gas
marked as the first gas) leaves the initial gas mix index at its default
value of zero. This is different from the OSTC2 compatible devices,
where a missing initial gas is stored as the value 0xFF.

By initializing the index with the value 0xFF, the existing detection
works for both variants.

henrik242/libdivecomputer,glance-/libdivecomputer,Poltsi/libdivecomputer-vms,glance-/libdivecomputer,henrik242/libdivecomputer