This has been here for a while: self_id hangs around while we're
calling the hook, but now it triggers sometimes.
```
E ValueError:
E Node errors:
E Global errors:
E - Node /tmp/ltests-3mcyp67u/test_dev_rawrequest_1/lightning-1/ has memory leaks: [
E {
E "backtrace": [
E "ccan/ccan/tal/tal.c:442 (tal_alloc_)",
E "gossipd/gossipd_wiregen.c:528 (fromwire_gossipd_got_onionmsg_to_us)",
E "lightningd/onion_message.c:152 (handle_onionmsg_to_us)",
E "lightningd/gossip_control.c:137 (gossip_msg)",
E "lightningd/subd.c:548 (sd_msg_read)",
E "ccan/ccan/io/io.c:59 (next_plan)",
E "ccan/ccan/io/io.c:407 (do_plan)",
E "ccan/ccan/io/io.c:417 (io_ready)",
E "ccan/ccan/io/poll.c:453 (io_loop)",
E "lightningd/io_loop_with_timers.c:21 (io_loop_with_timers)",
E "lightningd/lightningd.c:1164 (main)"
E ],
E "label": "gossipd/gossipd_wiregen.c:528:struct secret",
E "parents": [
E "lightningd/onion_message.c:149:struct onion_message_hook_payload",
E "lightningd/plugin_hook.c:81:struct hook_instance *[]"
E ],
E "value": "0x55cf3cbc9458"
E }
E ]
```