p2p: fix picking peers off an empty gray list

src/p2p/net_node.inl

@@ -1062,7 +1062,10 @@ namespace nodetool
         max_random_index = std::min<uint64_t>(local_peers_count -1, 20);
         random_index = get_random_index_with_fixed_probability(max_random_index);
       } else {
-        random_index = crypto::rand<size_t>() % m_peerlist.get_gray_peers_count();
+        local_peers_count = m_peerlist.get_gray_peers_count();
+        if (!local_peers_count)
+          return false;
+        random_index = crypto::rand<size_t>() % local_peers_count;
       }
 
       CHECK_AND_ASSERT_MES(random_index < local_peers_count, false, "random_starter_index < peers_local.size() failed!!");