try-fix: nodenum crash during boot

src/mesh/NodeDB.cpp

@@ -790,7 +790,8 @@ meshtastic_NodeInfoLite *NodeDB::getOrCreateMeshNode(NodeNum n)
 
     if (!lite) {
         if ((*numMeshNodes >= MAX_NUM_NODES) || (memGet.getFreeHeap() < meshtastic_NodeInfoLite_size * 3)) {
-            screen->print("warning: node_db_lite full! erasing oldest entry\n");
+            if (screen)
+                screen->print("warning: node_db_lite full! erasing oldest entry\n");
             // look for oldest node and erase it
             uint32_t oldest = UINT32_MAX;
             int oldestIndex = -1;