Without this, we can get errors on shutdown:
Valgrind error file: valgrind-errors.27444
==27444== Invalid read of size 8
==27444== at 0x1950E2: secp256k1_pubkey_load (secp256k1.c:127)
==27444== by 0x19CF87: secp256k1_ec_pubkey_serialize (secp256k1.c:189)
==27444== by 0x14FED9: towire_pubkey (towire.c:59)
==27444== by 0x15AAFB: towire_gossipctl_peer_disconnected (gen_gossip_wire.c:969)
==27444== by 0x1253EF: opening_channel_errmsg (opening_control.c:526)
==27444== by 0x1386A3: destroy_subd (subd.c:589)
==27444== by 0x18222C: notify (tal.c:240)
==27444== by 0x1826E1: del_tree (tal.c:400)
==27444== by 0x182733: del_tree (tal.c:410)
==27444== by 0x182733: del_tree (tal.c:410)
==27444== by 0x182B1F: tal_free (tal.c:511)
==27444== by 0x11FC53: main (lightningd.c:410)
==27444== Address 0x6c3af98 is 72 bytes inside a block of size 216 free'd
==27444== at 0x4C30D3B: free (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==27444== by 0x1827BC: del_tree (tal.c:421)
==27444== by 0x182B1F: tal_free (tal.c:511)
==27444== by 0x11F3C7: shutdown_subdaemons (lightningd.c:211)
==27444== by 0x11FC27: main (lightningd.c:406)
==27444== Block was alloc'd at
==27444== at 0x4C2FB0F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==27444== by 0x182296: allocate (tal.c:250)
==27444== by 0x182863: tal_alloc_ (tal.c:448)
==27444== by 0x12F2DF: new_peer (peer_control.c:74)
==27444== by 0x125600: new_uncommitted_channel (opening_control.c:576)
==27444== by 0x125870: peer_accept_channel (opening_control.c:668)
==27444== by 0x13032A: peer_sent_nongossip (peer_control.c:427)
==27444== by 0x116B9E: peer_nongossip (gossip_control.c:60)
==27444== by 0x116F2B: gossip_msg (gossip_control.c:172)
==27444== by 0x138323: sd_msg_read (subd.c:503)
==27444== by 0x137C02: read_fds (subd.c:330)
==27444== by 0x175550: next_plan (io.c:59)
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
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