Avoid global routing messing up FASM generation
diff --git a/utils/fasm/src/fasm.cpp b/utils/fasm/src/fasm.cpp
index 4f1becb..d6d1acc 100644
--- a/utils/fasm/src/fasm.cpp
+++ b/utils/fasm/src/fasm.cpp
@@ -605,6 +605,7 @@
 
     for(const auto &trace : route_ctx.trace) {
       t_trace *head = trace.head;
+      if (!head) continue;
       t_rt_node* root = traceback_to_route_tree(head);
       walk_route_tree(root);
       free_route_tree(root);