Fix two more potential crashes for dives without samples

This should be all of them (famous last words).

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

qt-ui/profile/diveeventitem.cpp

@@ -131,7 +131,7 @@ bool DiveEventItem::shouldBeHidden()
 	 * Don't bother showing those
 	 */
 	struct sample *first_sample = &get_dive_dc(&displayed_dive, dc_number)->sample[0];
-	if (!strcmp(event->name, "gaschange") && event->time.seconds == first_sample->time.seconds)
+	if (!strcmp(event->name, "gaschange") && first_sample && event->time.seconds == first_sample->time.seconds)
 		return true;
 
 	for (int i = 0; i < evn_used; i++) {