offer: make the blockheight nullable

However, it is possible to avoid this kind of crash, because
our assert is "wrong" because blockheight can be 0 and the
assert can fail.

In addition, the `rpc_scan` already checks for nullability, so the
assert is redundant

2024-06-07T14:14:29.278Z INFO    lightningd: v24.05-modded
2024-06-07T14:14:37.158Z INFO    lightningd: v24.05-modded
2024-06-07T14:14:29.329Z INFO    lightningd: Creating configuration directory /root/.lightning/signet
2024-06-07T14:14:37.589Z INFO    plugin-bcli: bitcoin-cli initialized and connected to bitcoind.
2024-06-07T14:14:29.715Z INFO    lightningd: Creating database
2024-06-07T14:14:37.591Z UNUSUAL lightningd: Waiting for initial block download (this can take a while!)
2024-06-07T14:14:29.752Z UNUSUAL hsmd: HSM: created new hsm_secret file
2024-06-07T14:14:37.603Z INFO    plugin-reckless-wrapper.py: initializing reckless-wrapper.py.
2024-06-07T14:14:29.798Z INFO    plugin-bcli: bitcoin-cli initialized and connected to bitcoind.
2024-06-07T14:14:29.802Z UNUSUAL lightningd: Waiting for initial block download (this can take a while!)
offers: plugins/offers.c:1235: init: Assertion `blockheight' failed.
2024-06-07T14:14:29.814Z INFO    plugin-reckless-wrapper.py: initializing reckless-wrapper.py.
offers: FATAL SIGNAL 6 (version v24.05-modded)
0x57ab55a56485 send_backtrace
2024-06-07T14:14:29.818Z INFO    plugin-chanbackup: Creating Emergency Recovery
   common/daemon.c:33
offers: plugins/offers.c:1235: init: Assertion `blockheight' failed.
offers: FATAL SIGNAL 6 (version v24.05-modded)
0x57ab55a5650d crashdump
   common/daemon.c:75
0x59f3ed3ba485 send_backtrace
0x7dd20b02213f ???
   common/daemon.c:33
   ???:0
0x59f3ed3ba50d crashdump
0x7dd20ae59ce1 ???
   common/daemon.c:75
   ???:0
0x7dd20ae43536 ???
0x764fa583813f ???
   ???:0
0x7dd20ae4340e ???
   ???:0
0x7dd20ae52661 ???
   ???:0
   ???:0
0x57ab55a3effe init
   plugins/offers.c:1235
0x764fa566fce1 ???
0x57ab55a4763f handle_init
   ???:0
   plugins/libplugin.c:1376
0x57ab55a478ee ld_command_handle
0x764fa5659536 ???
   plugins/libplugin.c:1797
0x57ab55a47ec0 ld_read_json_one
   plugins/libplugin.c:1979
   ???:0
0x57ab55a47f49 ld_read_json
0x764fa565940e ???
   plugins/libplugin.c:1999
   ???:0
0x57ab55b934cc next_plan
0x764fa5668661 ???
   ccan/ccan/io/io.c:60
   ???:0
0x57ab55b93953 do_plan
0x59f3ed3a2ffe init
   ccan/ccan/io/io.c:408
   plugins/offers.c:1235
0x57ab55b939ec io_ready
   ccan/ccan/io/io.c:418
0x59f3ed3ab63f handle_init
0x57ab55b95302 io_loop
   plugins/libplugin.c:1376
   ccan/ccan/io/poll.c:455
0x59f3ed3ab8ee ld_command_handle
0x57ab55a486dc plugin_main
   plugins/libplugin.c:1797
   plugins/libplugin.c:2209
0x59f3ed3abec0 ld_read_json_one
0x57ab55a3f36d main
   plugins/libplugin.c:1979
   plugins/offers.c:1285
0x59f3ed3abf49 ld_read_json
0x7dd20ae44d09 ???
   plugins/libplugin.c:1999
   ???:0
0x59f3ed4f74cc next_plan
0x57ab55a3c799 ???
   ccan/ccan/io/io.c:60
0x59f3ed4f7953 do_plan
   ???:0
   ccan/ccan/io/io.c:408
0xffffffffffffffff ???
0x59f3ed4f79ec io_ready
   ccan/ccan/io/io.c:418
   ???:0
offers: FATAL SIGNAL 11 (version v24.05-modded)
0x59f3ed4f9302 io_loop
   ccan/ccan/io/poll.c:455
0x57ab55a56485 send_backtrace
0x59f3ed3ac6dc plugin_main
   common/daemon.c:33
   plugins/libplugin.c:2209
0x57ab55a5650d crashdump
0x59f3ed3a336d main
   plugins/offers.c:1285
   common/daemon.c:75
0x7dd20b02213f ???
0x764fa565ad09 ???
   ???:0
   ???:0
0x59f3ed3a0799 ???
0x0 ???
   ???:0
   ???:0
2024-06-07T14:14:37.734Z INFO    plugin-offers: Killing plugin: exited before replying to init
0xffffffffffffffff ???
   ???:0
2024-06-07T14:14:37.734Z **BROKEN** plugin-offers: Plugin marked as important, shutting down lightningd!
offers: FATAL SIGNAL 11 (version v24.05-modded)
0x59f3ed3ba485 send_backtrace
   common/daemon.c:33
0x59f3ed3ba50d crashdump
   common/daemon.c:75
0x764fa583813f ???
   ???:0
0x0 ???
   ???:0
2024-06-07T14:14:29.823Z INFO    plugin-bookkeeper: Creating database
2024-06-07T14:14:29.883Z INFO    plugin-offers: Killing plugin: exited before replying to init
2024-06-07T14:14:29.883Z **BROKEN** plugin-offers: Plugin marked as important, shutting down lightningd!

Fixes: 9d75fbe237
Reported-by: @farscapian
Link: https://github.com/ElementsProject/lightning/issues/7378
Signed-off-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>

plugins/offers.c

@@ -1232,7 +1232,6 @@ static const char *init(struct plugin *p,
 	rpc_scan(p, "getchaininfo",
 		 take(json_out_obj(NULL, "last_height", NULL)),
 		 "{headercount:%}", JSON_SCAN(json_to_u32, &blockheight));
-	assert(blockheight);
 
 	rpc_scan(p, "listconfigs",
 		 take(json_out_obj(NULL, NULL, NULL)),