Don't access invalid elements with depth = 0

This is a corner case in the planner that was exposed by the recent
changes to the way the dive plan reflects the gases during the dive.

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

1 files changed, 2 insertions, 1 deletions

diff --git a/planner.c b/planner.c
index abefb6213..687da8273 100644
--- a/planner.c
+++ b/planner.c
@@ -616,7 +616,8 @@ void plan(struct diveplan *diveplan, char **cached_datap, struct dive **divep, b
 	for (stopidx = 0; stopidx < sizeof(decostoplevels) / sizeof(int); stopidx++)
 		if (decostoplevels[stopidx] >= depth)
 			break;
-	stopidx--;
+	if (stopidx > 0)
+		stopidx--;
 
 	/* so now we know the first decostop level above us
 	 * NOTE, this could be the surface or a long list of potential stops