[repacker] enforce root node having no incoming edges.

src/graph/graph.hh

@@ -995,6 +995,10 @@ struct graph_t
   {
     update_parents();
 
+    if (root().parents)
+      // Root cannot have parents.
+      return false;
+
     for (unsigned i = 0; i < root_idx (); i++)
     {
       if (!vertices_[i].parents)
@@ -1061,6 +1065,10 @@ struct graph_t
     parents_invalid = true;
     update_parents();
 
+    if (root().parents) {
+      DEBUG_MSG (SUBSET_REPACK, nullptr, "Root node has incoming edges.");
+    }
+
     for (unsigned i = 0; i < root_idx (); i++)
     {
       const auto& v = vertices_[i];