lightningd: fix crash on dynamic plugin startup

2019-07-29T04:25:31.503Z DEBUG plugin-manager started(25413) /home/rusty/text/webinar/lightning-2019-07/helloworld.py
2019-07-29T04:25:39.168Z UNUSUAL lightningd(24972): Unable to estimate CONSERVATIVE/2 fee
2019-07-29T04:25:39.193Z **BROKEN** lightningd(24972): FATAL SIGNAL 6 (version v0.7.1-144-g6bb8525)
2019-07-29T04:25:39.193Z **BROKEN** lightningd(24972): backtrace: common/daemon.c:45 (send_backtrace) 0x5581b809d7d6
2019-07-29T04:25:39.193Z **BROKEN** lightningd(24972): backtrace: common/daemon.c:53 (crashdump) 0x5581b809d823
2019-07-29T04:25:39.193Z **BROKEN** lightningd(24972): backtrace: (null):0 ((null)) 0x7f4ecd46af5f
2019-07-29T04:25:39.193Z **BROKEN** lightningd(24972): backtrace: (null):0 ((null)) 0x7f4ecd46aed7
2019-07-29T04:25:39.193Z **BROKEN** lightningd(24972): backtrace: (null):0 ((null)) 0x7f4ecd44c534
2019-07-29T04:25:39.193Z **BROKEN** lightningd(24972): backtrace: ccan/ccan/tal/tal.c:93 (call_error) 0x5581b80da741
2019-07-29T04:25:39.193Z **BROKEN** lightningd(24972): backtrace: ccan/ccan/tal/tal.c:165 (check_bounds) 0x5581b80da81d
2019-07-29T04:25:39.193Z **BROKEN** lightningd(24972): backtrace: ccan/ccan/tal/tal.c:174 (to_tal_hdr) 0x5581b80da845
2019-07-29T04:25:39.193Z **BROKEN** lightningd(24972): backtrace: ccan/ccan/tal/tal.c:186 (to_tal_hdr_or_null) 0x5581b80da88e
2019-07-29T04:25:39.193Z **BROKEN** lightningd(24972): backtrace: ccan/ccan/tal/tal.c:632 (tal_bytelen) 0x5581b80db59c
2019-07-29T04:25:39.193Z **BROKEN** lightningd(24972): backtrace: lightningd/plugin.c:1056 (plugin_subscriptions_contains) 0x5581b8095aca
2019-07-29T04:25:39.193Z **BROKEN** lightningd(24972): backtrace: lightningd/plugin.c:1071 (plugins_notify) 0x5581b80972f4
2019-07-29T04:25:39.193Z **BROKEN** lightningd(24972): backtrace: lightningd/notification.c:63 (notify_warning) 0x5581b808615a
2019-07-29T04:25:39.193Z **BROKEN** lightningd(24972): backtrace: lightningd/log.c:269 (logv) 0x5581b8085393
2019-07-29T04:25:39.193Z **BROKEN** lightningd(24972): backtrace: lightningd/log.c:329 (log_) 0x5581b8085233
2019-07-29T04:25:39.193Z **BROKEN** lightningd(24972): backtrace: lightningd/bitcoind.c:372 (process_estimatefee) 0x5581b80759d7
2019-07-29T04:25:39.193Z **BROKEN** lightningd(24972): backtrace: lightningd/bitcoind.c:226 (bcli_finished) 0x5581b8075cfb
2019-07-29T04:25:39.193Z **BROKEN** lightningd(24972): backtrace: ccan/ccan/io/poll.c:244 (destroy_conn) 0x5581b80d2413
2019-07-29T04:25:39.193Z **BROKEN** lightningd(24972): backtrace: ccan/ccan/io/poll.c:250 (destroy_conn_close_fd) 0x5581b80d2435
2019-07-29T04:25:39.193Z **BROKEN** lightningd(24972): backtrace: ccan/ccan/tal/tal.c:235 (notify) 0x5581b80dabb0
2019-07-29T04:25:39.193Z **BROKEN** lightningd(24972): backtrace: ccan/ccan/tal/tal.c:397 (del_tree) 0x5581b80dac7a
2019-07-29T04:25:39.193Z **BROKEN** lightningd(24972): backtrace: ccan/ccan/tal/tal.c:481 (tal_free) 0x5581b80db1c5
2019-07-29T04:25:39.193Z **BROKEN** lightningd(24972): backtrace: ccan/ccan/io/io.c:450 (io_close) 0x5581b80d1147
2019-07-29T04:25:39.193Z **BROKEN** lightningd(24972): backtrace: ccan/ccan/io/poll.c:449 (io_loop) 0x5581b80d29ff
2019-07-29T04:25:39.193Z **BROKEN** lightningd(24972): backtrace: lightningd/io_loop_with_timers.c:24 (io_loop_with_timers) 0x5581b808058d
2019-07-29T04:25:39.193Z **BROKEN** lightningd(24972): backtrace: lightningd/lightningd.c:830 (main) 0x5581b8084396
2019-07-29T04:25:39.193Z **BROKEN** lightningd(24972): backtrace: (null):0 ((null)) 0x7f4ecd44db6a
2019-07-29T04:25:39.193Z **BROKEN** lightningd(24972): backtrace: (null):0 ((null)) 0x5581b8075059
2019-07-29T04:25:39.193Z **BROKEN** lightningd(24972): backtrace: (null):0 ((null)) 0xffffffffffffffff

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

lightningd/plugin.c

@@ -69,6 +69,7 @@ void plugin_register(struct plugins *plugins, const char* path TAKES)
 	p->configured = false;
 	p->js_arr = tal_arr(p, struct json_stream *, 0);
 	p->used = 0;
+	p->subscriptions = NULL;
 	p->signal_startup = false;
 
 	p->log = new_log(p, plugins->log_book, "plugin-%s",