fix: fixes FATAL SIGNAL 11 on gossmap node

This will fix a crash that I caused on armv7
and by looking inside the coredump with gdb
(by adding an assert on n that must be
different from null) I get the following stacktrace

```
(gdb) bt
\#0  0x00000000 in ?? ()
\#1  0x0043a038 in send_backtrace (why=0xbe9e3600 "FATAL SIGNAL 11") at common/daemon.c:36
\#2  0x0043a0ec in crashdump (sig=11) at common/daemon.c:46
\#3  <signal handler called>
\ElementsProject#4  0x00406d04 in node_announcement (map=0x938ecc, nann_off=495146) at common/gossmap.c:586
\ElementsProject#5  0x00406fec in map_catchup (map=0x938ecc, num_rejected=0xbe9e3a40) at common/gossmap.c:643
\ElementsProject#6  0x004073a4 in load_gossip_store (map=0x938ecc, num_rejected=0xbe9e3a40) at common/gossmap.c:697
\ElementsProject#7  0x00408244 in gossmap_load (ctx=0x0, filename=0x4e16b8 "gossip_store", num_channel_updates_rejected=0xbe9e3a40) at common/gossmap.c:976
\ElementsProject#8  0x0041a548 in init (p=0x93831c, buf=0x9399d4 "\n\n{\"jsonrpc\":\"2.0\",\"id\":\"cln:init#25\",\"method\":\"init\",\"params\":{\"options\":{},\"configuration\":{\"lightning-dir\":\"/home/vincent/.lightning/testnet\",\"rpc-file\":\"lightning-rpc\",\"startup\":true,\"network\":\"te"..., config=0x939cdc) at plugins/topology.c:622
\ElementsProject#9  0x0041e5d0 in handle_init (cmd=0x938934, buf=0x9399d4 "\n\n{\"jsonrpc\":\"2.0\",\"id\":\"cln:init#25\",\"method\":\"init\",\"params\":{\"options\":{},\"configuration\":{\"lightning-dir\":\"/home/vincent/.lightning/testnet\",\"rpc-file\":\"lightning-rpc\",\"startup\":true,\"network\":\"te"..., params=0x939c8c)
    at plugins/libplugin.c:1208
\ElementsProject#10 0x0041fc04 in ld_command_handle (plugin=0x93831c, toks=0x939bec) at plugins/libplugin.c:1572
\ElementsProject#11 0x00420050 in ld_read_json_one (plugin=0x93831c) at plugins/libplugin.c:1667
\ElementsProject#12 0x004201bc in ld_read_json (conn=0x9391c4, plugin=0x93831c) at plugins/libplugin.c:1687
\ElementsProject#13 0x004cb82c in next_plan (conn=0x9391c4, plan=0x9391d8) at ccan/ccan/io/io.c:59
\ElementsProject#14 0x004cc67c in do_plan (conn=0x9391c4, plan=0x9391d8, idle_on_epipe=false) at ccan/ccan/io/io.c:407
\ElementsProject#15 0x004cc6dc in io_ready (conn=0x9391c4, pollflags=1) at ccan/ccan/io/io.c:417
\ElementsProject#16 0x004cf8cc in io_loop (timers=0x9383c4, expired=0xbe9e3ce4) at ccan/ccan/io/poll.c:453
\ElementsProject#17 0x00420af4 in plugin_main (argv=0xbe9e3eb4, init=0x41a46c <init>, restartability=PLUGIN_STATIC, init_rpc=true, features=0x0, commands=0x6167e8 <commands>, num_commands=4, notif_subs=0x0, num_notif_subs=0, hook_subs=0x0, num_hook_subs=0, notif_topics=0x0, num_notif_topics=0) at plugins/libplugin.c:1891
\ElementsProject#18 0x0041a6f8 in main (argc=1, argv=0xbe9e3eb4) at plugins/topology.c:679
```

I do not know if this is a solution because I do not know
when I can parse a node announcement for a node that
it is not longer in the gossip map.

So, I hope this is just usefult for @rustyrussell

Changelog-Fixed: fixes `FATAL SIGNAL 11` on gossmap node announcement parsing.

Signed-off-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>

common/gossmap.c

@@ -582,8 +582,8 @@ static void node_announcement(struct gossmap *map, size_t nann_off)
 
 	feature_len = map_be16(map, nann_off + feature_len_off);
 	map_nodeid(map, nann_off + feature_len_off + 2 + feature_len + 4, &id);
-	n = gossmap_find_node(map, &id);
-	n->nann_off = nann_off;
+	if ((n = gossmap_find_node(map, &id)))
+		n->nann_off = nann_off;
 }
 
 static void reopen_store(struct gossmap *map, size_t ended_off)