Fix GC catastrophe in Slur_proto_engraver::derived_mark

In Slur_proto_engraver::Event_info, note_ will be 0 for slurs not
attached to particular note events.  Calling self_scm on it is likely
to be a fatal error.

diff --git a/lily/slur-proto-engraver.cc b/lily/slur-proto-engraver.cc
index 5accd2c073fddb9fcd4f4f1063f5e434e415bb2c..99e8e8ebfcd54fd87f8ac370466e01f0b45b82ad 100644 (file)
--- a/lily/slur-proto-engraver.cc
+++ b/lily/slur-proto-engraver.cc
@@ -38,12 +38,14 @@ Slur_proto_engraver::derived_mark () const
   for (vsize i = start_events_.size (); i--;)
     {
       scm_gc_mark (start_events_[i].slur_->self_scm ());
-      scm_gc_mark (start_events_[i].note_->self_scm ());
+      if (start_events_[i].note_)
+        scm_gc_mark (start_events_[i].note_->self_scm ());
     }
   for (vsize i = stop_events_.size (); i--;)
     {
       scm_gc_mark (stop_events_[i].slur_->self_scm ());
-      scm_gc_mark (stop_events_[i].note_->self_scm ());
+      if (stop_events_[i].note_)
+        scm_gc_mark (stop_events_[i].note_->self_scm ());
     }
 }