gossmap: ensure chan not null

[JssDWt]

Ignore localmods that don't have a corresponding entry in the gossmap.

A crash was observed on this branch: https://github.com/breez/lightning/tree/cln-v24.08-breez with commit https://github.com/breez/lightning/commit/bc9e4f56c324216f5f0f15be07f6ad4f9a46e597

pay: FATAL SIGNAL 11 (version v24.08-4-gbc9e4f5-modded)
0x5584c2da9cbf send_backtrace
        common/daemon.c:33
0x5584c2da9d44 crashdump
        common/daemon.c:75
0x7fc69664858f ???
        ???:0
0x5584c2dc2864 gossmap_remove_localmods
        common/gossmap.c:984
0x5584c2d94b2f put_gossmap
        plugins/libplugin-pay.c:62
0x5584c2d9ac32 routehint_step_cb
        plugins/libplugin-pay.c:3171
0x5584c2d98fda payment_continue
        plugins/libplugin-pay.c:2450
0x5584c2d99928 shadow_route_cb
        plugins/libplugin-pay.c:3529
0x5584c2d98fda payment_continue
        plugins/libplugin-pay.c:2450
0x5584c2d9b585 direct_pay_override
        plugins/libplugin-pay.c:3550
0x5584c2d9b7a8 direct_pay_listpeerchannels
        plugins/libplugin-pay.c:3621
0x5584c2d93713 handle_rpc_reply
        plugins/libplugin.c:1016
0x5584c2d938b7 rpc_read_response_one
        plugins/libplugin.c:1202
0x5584c2d93964 rpc_conn_read_response
        plugins/libplugin.c:1226
0x5584c2ef37cc next_plan
        ccan/ccan/io/io.c:60
0x5584c2ef3c57 do_plan
        ccan/ccan/io/io.c:422
0x5584c2ef3d10 io_ready
        ccan/ccan/io/io.c:439
0x5584c2ef55fc io_loop
        ccan/ccan/io/poll.c:455
0x5584c2d94006 plugin_main
        plugins/libplugin.c:2230
0x5584c2d8f029 main
        plugins/pay.c:1533
0x7fc696632c89 ???
        ???:0
0x7fc696632d44 ???
        ???:0
0x5584c2d8b7b0 ???
        ???:0
0xffffffffffffffff ???
        ???:0

The branch contains changes compared to v24.08, namely

• https://github.com/ElementsProject/lightning/pull/7628
• https://github.com/ElementsProject/lightning/pull/7611
• https://github.com/ElementsProject/lightning/pull/7636

But I don't think they were related to the crash. A simple null check should suffice here?

Checklist

Before submitting the PR, ensure the following tasks are completed. If an item is not applicable to your PR, please mark it as checked:

• [x] The changelog has been updated in the relevant commit(s) according to the guidelines.
• [ ] Tests have been added or modified to reflect the changes.
• [ ] Documentation has been reviewed and updated as needed.
• [ ] Related issues have been listed and linked, including any that this PR closes.