midx-write: fix segfault and do several cleanups #24562

Job	Run time
config	6s
fuzz smoke test	1m 20s
win+Meson build	3m 21s
win build	4m 32s
osx-meson (macos-13)	36m 56s
osx-gcc (macos-13)	26m 16s
static-analysis	4m 57s
linux-meson (ubuntu:rolling)	12m 12s
osx-clang (macos-13)	30m 23s
documentation	9m 20s
linux-sha256 (ubuntu:rolling)	11m 29s
linux-asan-ubsan (ubuntu:rolling)	32m 56s
linux-TEST-vars (ubuntu:20.04)	12m 40s
linux-reftable (ubuntu:rolling)	11m 44s
almalinux-8 (almalinux:8)	9m 14s
debian-11 (debian:11)	10m 1s
linux-leaks (ubuntu:rolling)	32m 5s
osx-reftable (macos-13)	34m 31s
linux-breaking-changes (ubuntu:rolling)	10m 49s
sparse	3m 36s
linux32 (i386/ubuntu:focal)	9m 47s
pedantic (fedora:latest)	1m 52s
linux-reftable-leaks (ubuntu:rolling)	33m 26s
linux-musl-meson (alpine:latest)	8m 0s
win+VS build	0s
win+VS test	0s
win+Meson test (1)	9m 0s
win+Meson test (0)	7m 47s
win+Meson test (7)	6m 37s
win+Meson test (8)	8m 32s
win+Meson test (3)	7m 3s
win+Meson test (9)	8m 34s
win+Meson test (2)	6m 27s
win+Meson test (6)	8m 6s
win+Meson test (5)	6m 2s
win+Meson test (4)	10m 1s
win test (0)	6m 32s
win test (1)	6m 54s
win test (5)	5m 35s
win test (4)	7m 48s
win test (3)	3m 59s
win test (9)	5m 57s
win test (7)	5m 42s
win test (8)	7m 43s
win test (6)	5m 10s
win test (2)	5m 8s
	8h 10m 10s

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