vpr: added check for null pointer dereference in route_tree_timing.cpp

vpr/src/route/route_tree_timing.cpp

diff --git a/vpr/src/route/route_tree_timing.cpp b/vpr/src/route/route_tree_timing.cpp
index 7631b32..ca69f93 100644
--- a/vpr/src/route/route_tree_timing.cpp
+++ b/vpr/src/route/route_tree_timing.cpp
@@ -1426,7 +1426,7 @@
 
     for (t_linked_rt_edge* edge = node->u.child_list; edge != nullptr; edge = edge->next) {
         found_node = find_sink_rt_node_recurr(edge->child, sink_rr_inode); //process each of the children
-
+        VTR_ASSERT(found_node);
         if (found_node->inode == sink_rr_inode) {
             //If the sink has been found downstream in the branch, we would like to immediately exit the search
             return found_node;