gossipd: make sure we never write bad entries.

We have reports of crashes on reading gossip_store, including from gossipd itself!

```
lightning_gossipd: common/gossmap.c:121: map_copy: Assertion `offset + len <= map->map_size' failed.
...
lightning_gossipd: FATAL SIGNAL (version v24.11)
0x6260c41d682a send_backtrace
  common/daemon.c:33
0x6260c41e098b status_failed
  common/status.c:221
0x6260c41e0b41 status_backtrace_exit
  common/subdaemon.c:18
0x6260c41d68b8 crashdump
  common/daemon.c:78
0x70508ea6913f ???
  ???:0
0x70508e8a0d51 ???
  ???:0
0x70508e88a536 ???
  ???:0
0x70508e88a40e ???
  ???:0
0x70508e8996d1 ???
  ???:0
0x6260c41d8b69 map_copy
  common/gossmap.c:121
0x6260c41d8bab map_be16
  common/gossmap.c:142
0x6260c41daa45 map_catchup
  common/gossmap.c:705
0x6260c41dab95 gossmap_refresh_mayfail
  common/gossmap.c:1192
0x6260c41daca6 gossmap_refresh
  common/gossmap.c:1213
0x6260c41cee32 gossmap_manage_get_gossmap
  gossipd/gossmap_manage.c:1314
0x6260c41d0686 gossmap_manage_new_block
  gossipd/gossmap_manage.c:1221
0x6260c41cbfdd new_blockheight
  gossipd/gossipd.c:473
0x6260c41cc363 recv_req
  gossipd/gossipd.c:584
0x6260c41d6b1d handle_read
  common/daemon_conn.c:35
0x6260c43175b5 next_plan
  ccan/ccan/io/io.c:60
0x6260c4317a40 do_plan
  ccan/ccan/io/io.c:422
0x6260c4317af9 io_ready
  ccan/ccan/io/io.c:439
0x6260c4319446 io_loop
  ccan/ccan/io/poll.c:455
0x6260c41cccf4 main
  gossipd/gossipd.c:665
```

This implies that we have a message shorter than 2 bytes, which should never happen.

An audit didn't shed any light, but let's make sure we don't ever write such a thing.

Signed-off-by: Rusty Russell <[email protected]>

gossipd/gossip_store.c

@@ -70,7 +70,11 @@ static bool append_msg(int fd, const u8 *msg, u32 timestamp, u64 *len)
 	/* Don't ever overwrite the version header! */
 	assert(*len);
 
+	/* Never NULL */
+	assert(msg);
 	msglen = tal_count(msg);
+	/* All messages begin with a 16-bit type */
+	assert(msglen >= 2);
 	hdr.len = cpu_to_be16(msglen);
 	hdr.flags = 0;
 	hdr.crc = cpu_to_be32(crc32c(timestamp, msg, msglen));