`spliceout` causes MEMLEAK if more that current channel capacity is spliced out during shutdown

[enaples]

Here the logs

lightningd-1 2026-04-09T15:12:21.712Z **BROKEN** plugin-spenderp: MEMLEAK: 0xc5a801228
lightningd-1 2026-04-09T15:12:21.712Z **BROKEN** plugin-spenderp:   label=struct abort_pkg
lightningd-1 2026-04-09T15:12:21.712Z **BROKEN** plugin-spenderp:   alloc:
lightningd-1 2026-04-09T15:12:21.712Z **BROKEN** plugin-spenderp:     plugins/spender/splice.c:190 (do_fail)
lightningd-1 2026-04-09T15:12:21.712Z **BROKEN** plugin-spenderp:     plugins/libplugin.c:0 (handle_rpc_reply)
lightningd-1 2026-04-09T15:12:21.712Z **BROKEN** plugin-spenderp:     plugins/libplugin.c:1398 (rpc_conn_read_response)
lightningd-1 2026-04-09T15:12:21.712Z **BROKEN** plugin-spenderp:     ccan/ccan/io/io.c:60 (next_plan)
lightningd-1 2026-04-09T15:12:21.712Z **BROKEN** plugin-spenderp:     ccan/ccan/io/io.c:422 (do_plan)
lightningd-1 2026-04-09T15:12:21.712Z **BROKEN** plugin-spenderp:     ccan/ccan/io/io.c:439 (io_ready)
lightningd-1 2026-04-09T15:12:21.712Z **BROKEN** plugin-spenderp:     ccan/ccan/io/poll.c:470 (io_loop)
lightningd-1 2026-04-09T15:12:21.712Z **BROKEN** plugin-spenderp:     plugins/libplugin.c:2461 (plugin_main)
lightningd-1 2026-04-09T15:12:21.712Z **BROKEN** plugin-spenderp:     plugins/spender/main.c:37 (main)
lightningd-1 2026-04-09T15:12:21.712Z **BROKEN** plugin-spenderp:   parents:
lightningd-1 2026-04-09T15:12:21.712Z **BROKEN** plugin-spenderp:     struct plugin