Prevent segfault for dive with gas change but no samples

Not quite sure where such a dive would come from, but anyway, just don't
dereference the pointer unless it's non-NULL.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

1 files changed, 1 insertions, 1 deletions

diff --git a/dive.c b/dive.c
index 5bcdf3aa8..73c7bd2e7 100644
--- a/dive.c
+++ b/dive.c
@@ -789,7 +789,7 @@ static int same_rounded_pressure(pressure_t a, pressure_t b)
 int explicit_first_cylinder(struct dive *dive, struct divecomputer *dc)
 {
 	struct event *ev = get_next_event(dc->events, "gaschange");
-	if (ev && ev->time.seconds == dc->sample[0].time.seconds)
+	if (ev && dc && dc->sample && ev->time.seconds == dc->sample[0].time.seconds)
 		return get_cylinder_index(dive, ev);
 	else
 		return 0;