We cannot carry pointers into the gossmap across localmod addition
or removal.
We didn't notice because the map->chan_arr is not normally resized,
but if we change gossmap.c line 689 to only allocate 1 to start, we see this:
```
VALGRIND=1 valgrind -q --error-exitcode=7 --track-origins=yes --leak-check=full --show-reachable=yes --errors-for-leak-kinds=all plugins/renepay/test/run-mcf > /dev/null
==2349744== Invalid read of size 4
==2349744== at 0x1788C2: gossmap_chan_scid (gossmap.c:558)
==2349744== by 0x1872A2: get_chan_extra_half_by_chan (flow.c:346)
==2349744== by 0x187797: remove_completed_flow (flow.c:488)
==2349744== by 0x187927: remove_completed_flow_set (flow.c:518)
==2349744== by 0x18DF4D: main (run-mcf.c:393)
==2349744== Address 0x4b80f38 is 88 bytes inside a block of size 136 free'd
==2349744== at 0x4848C63: realloc (in /usr/libexec/valgrind/vgpreload_memcheck-amd64-linux.so)
==2349744== by 0x173D71: tal_resize_ (tal.c:744)
==2349744== by 0x177E36: next_free_chan (gossmap.c:336)
==2349744== by 0x177ED3: new_channel (gossmap.c:351)
==2349744== by 0x178441: add_channel (gossmap.c:458)
==2349744== by 0x1798D4: gossmap_apply_localmods (gossmap.c:904)
==2349744== by 0x18DEDB: main (run-mcf.c:388)
==2349744== Block was alloc'd at
==2349744== at 0x4848C63: realloc (in /usr/libexec/valgrind/vgpreload_memcheck-amd64-linux.so)
==2349744== by 0x173D71: tal_resize_ (tal.c:744)
==2349744== by 0x177E36: next_free_chan (gossmap.c:336)
==2349744== by 0x177ED3: new_channel (gossmap.c:351)
==2349744== by 0x178441: add_channel (gossmap.c:458)
==2349744== by 0x178B6D: map_catchup (gossmap.c:635)
==2349744== by 0x178F45: load_gossip_store (gossmap.c:697)
==2349744== by 0x179D71: gossmap_load (gossmap.c:978)
==2349744== by 0x18D22F: main (run-mcf.c:295)
```
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
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