[gitg] Collapse only existing lanes

[Alberto Fanjul <albfan src gnome org>]

commit 41c647fd9c5eeaa9d51fdaca2e281332b3f5082a
Author: Alberto Fanjul <albertofanjul gmail com>
Date:   Wed Jan 2 22:45:46 2019 +0100

    Collapse only existing lanes

 libgitg/gitg-lanes.vala | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/libgitg/gitg-lanes.vala b/libgitg/gitg-lanes.vala
index 1af72e05..d782bba9 100644
--- a/libgitg/gitg-lanes.vala
+++ b/libgitg/gitg-lanes.vala
@@ -334,7 +334,7 @@ public class Lanes : Object
                        var commit = item.data;
                        unowned SList<Lane> lns = commit.get_lanes();
 
-                       if (lns != null)
+                       if (lns != null && index <= lns.length())
                        {
                                unowned Lane lane = lns.nth_data(index);