QBONE Bonnet: Ensure `eth_hdr_` is initialized when sending neighbor solicit responses.

The initialization was moved to WritePacket, but it's possible that the device reads a neighbor solicitation packet before any inbound packets have been written.

Caught by memory sanitizer run of a unit test that only reads packets.

This now passes `blaze test --config=msan//third_party/quic/qbone/bonnet:tun_device_packet_exchanger_test`

PiperOrigin-RevId: 941117920

quiche/quic/qbone/bonnet/tun_device_packet_exchanger.cc

diff --git a/quiche/quic/qbone/bonnet/tun_device_packet_exchanger.cc b/quiche/quic/qbone/bonnet/tun_device_packet_exchanger.cc
index d15c38c..7575359 100644
--- a/quiche/quic/qbone/bonnet/tun_device_packet_exchanger.cc
+++ b/quiche/quic/qbone/bonnet/tun_device_packet_exchanger.cc
@@ -185,6 +185,10 @@
   }
 
   if (is_neighbor_solicit) {
+    // We need the local interface MAC address to respond.
+    if (!eth_hdr_initialized_) {
+      InitializeEthHdr();
+    }
     // If we've received a neighbor solicitation, craft an advertisement to
     // respond with and write it back to the local interface.
     auto* icmp6_payload = packet.data() + kIcmp6PrefixLen;