Correctly handle first gas when deciding if gas is used

Gas with index 0 is assumed to be the first gas only if there is no
gaschage event in the first 30 seconds of the dive.

[Dirk Hohndel: minor formatting change in a logical expression]

Signed-off-by: Robert C. Helling <helling@atdotde.de>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>