lightningd: fix reconnect while openingd still live.

In this case, we unset the old subd->peer, then freed subd.
peer_owner_finished dereferenced subd->peer->owner, and boom:

test_disconnect_funder (__main__.LightningDTests) ... Fatal signal 11. Log dumped in crash.log

------------------------------- Valgrind errors --------------------------------
Valgrind error file: valgrind-errors.2882
==2882== Invalid read of size 8
==2882==    at 0x413F74: peer_owner_finished (peer_control.c:679)
==2882==    by 0x41EA2C: destroy_subd (subd.c:381)
==2882==    by 0x459700: notify (tal.c:240)
==2882==    by 0x459BB1: del_tree (tal.c:400)
==2882==    by 0x459FC0: tal_free (tal.c:509)
==2882==    by 0x413796: peer_reconnected (peer_control.c:493)
==2882==    by 0x413A6A: add_peer (peer_control.c:592)
==2882==    by 0x40ED1F: handshake_succeeded (new_connection.c:186)
==2882==    by 0x41E3DD: sd_msg_reply (subd.c:262)
==2882==    by 0x41E6BB: sd_msg_read (subd.c:318)
==2882==    by 0x41E4E6: read_fds (subd.c:283)
==2882==    by 0x44DEB4: next_plan (io.c:59)
==2882==  Address 0x838 is not stack'd, malloc'd or (recently) free'd
==2882==

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

lightningd/peer_control.c

@@ -490,7 +490,7 @@ static bool peer_reconnected(struct lightningd *ld,
 
 	case OPENINGD:
 		/* Kill off openingd, forget old peer. */
-		peer->owner->peer = NULL;
+		peer->owner = NULL; /* We'll free it ourselves */
 		tal_free(peer->owner);
 		tal_free(peer);