Fix bad ack

quic/core/quic_packet_generator.cc

diff --git a/quic/core/quic_packet_generator.cc b/quic/core/quic_packet_generator.cc
index d3d950c..096f06a 100644
--- a/quic/core/quic_packet_generator.cc
+++ b/quic/core/quic_packet_generator.cc
@@ -523,6 +523,10 @@
                                          NOT_HANDSHAKE)) {
       return false;
     }
+    if (frame.type == ACK_FRAME && frame.ack_frame->packets.Empty()) {
+      QUIC_LOG(ERROR) << "Bad ACK!!!";
+      continue;
+    }
     const bool success =
         packet_creator_.AddSavedFrame(frame, next_transmission_type_);
     QUIC_BUG_IF(!success) << "Failed to flush " << frame;