gossmap: fix bug in gossmap code

Updating ccan to stricter htable (as in current ccan!) revealed we were trying to put
(void *)1 in the htable, which is forbidden:

```
topology: ccan/ccan/htable/htable.c:382: htable_add_: Assertion `entry_is_valid((uintptr_t)p)' failed.
topology: FATAL SIGNAL 6 (version 1358d7f)
0x55f30c689c34 send_backtrace
	common/daemon.c:33
0x55f30c689ce0 crashdump
	common/daemon.c:46
0x7f5d150fe51f ???
	./signal/../sysdeps/unix/sysv/linux/x86_64/libc_sigaction.c:0
0x7f5d15152828 __pthread_kill_implementation
	./nptl/pthread_kill.c:44
0x7f5d15152828 __pthread_kill_internal
	./nptl/pthread_kill.c:80
0x7f5d15152828 __GI___pthread_kill
	./nptl/pthread_kill.c:91
0x7f5d150fe475 __GI_raise
	../sysdeps/posix/raise.c:26
0x7f5d150e47b6 __GI_abort
	./stdlib/abort.c:79
0x7f5d150e46da __assert_fail_base
	./assert/assert.c:92
0x7f5d150f5e25 __GI___assert_fail
	./assert/assert.c:101
0x55f30c6adbe4 htable_add_
	ccan/ccan/htable/htable.c:382
0x55f30c65f303 chanidx_htable_add
	common/gossmap.c:35
0x55f30c6605ed new_channel
	common/gossmap.c:337
0x55f30c6609cf add_channel
	common/gossmap.c:425
0x55f30c661101 map_catchup
	common/gossmap.c:607
0x55f30c66221e gossmap_refresh
	common/gossmap.c:927
0x55f30c66e3e9 get_gossmap
	plugins/topology.c:27
0x55f30c66f939 listpeers_done
	plugins/topology.c:369
0x55f30c671f46 handle_rpc_reply
	plugins/libplugin.c:558
0x55f30c672a19 rpc_read_response_one
	plugins/libplugin.c:726
0x55f30c672b4f rpc_conn_read_response
	plugins/libplugin.c:746
0x55f30c6ae35e next_plan
	ccan/ccan/io/io.c:59
0x55f30c6aef93 do_plan
	ccan/ccan/io/io.c:407
0x55f30c6aefd5 io_ready
	ccan/ccan/io/io.c:417
0x55f30c6b1371 io_loop
	ccan/ccan/io/poll.c:453
0x55f30c67587c plugin_main
	plugins/libplugin.c:1559
0x55f30c6708eb main
	plugins/topology.c:701
0x7f5d150e5fcf __libc_start_call_main
	../sysdeps/nptl/libc_start_call_main.h:58
0x7f5d150e607c __libc_start_main_impl
	../csu/libc-start.c:409
0x55f30c65d894 ???
	???:0
0xffffffffffffffff ???
	???:0
```

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

common/gossmap.c

@@ -176,25 +176,25 @@ u32 gossmap_chan_idx(const struct gossmap *map, const struct gossmap_chan *chan)
 	return chan - map->chan_arr;
 }
 
-/* htable can't handle NULL values, so we add 1 */
+/* htable can't handle NULL or 1 values, so we add 2 */
 static struct gossmap_chan *ptrint2chan(const ptrint_t *pidx)
 {
-	return map->chan_arr + ptr2int(pidx) - 1;
+	return map->chan_arr + ptr2int(pidx) - 2;
 }
 
 static ptrint_t *chan2ptrint(const struct gossmap_chan *chan)
 {
-	return int2ptr(chan - map->chan_arr + 1);
+	return int2ptr(chan - map->chan_arr + 2);
 }
 
 static struct gossmap_node *ptrint2node(const ptrint_t *pidx)
 {
-	return map->node_arr + ptr2int(pidx) - 1;
+	return map->node_arr + ptr2int(pidx) - 2;
 }
 
 static ptrint_t *node2ptrint(const struct gossmap_node *node)
 {
-	return int2ptr(node - map->node_arr + 1);
+	return int2ptr(node - map->node_arr + 2);
 }
 
 static struct short_channel_id chanidx_id(const ptrint_t *pidx)