midx-write: fix segfault and do several cleanups #24606

Job	Run time
config	6s
fuzz smoke test	2m 16s
win+VS build	0s
win build	4m 42s
osx-reftable (macos-13)	43m 44s
osx-clang (macos-13)	37m 6s
documentation	8m 59s
static-analysis	5m 5s
osx-gcc (macos-13)	37m 18s
sparse	3m 22s
win+Meson build	3m 22s
osx-meson (macos-13)	44m 59s
linux32 (i386/ubuntu:focal)	10m 8s
linux-asan-ubsan (ubuntu:rolling)	32m 37s
pedantic (fedora:latest)	1m 51s
linux-leaks (ubuntu:rolling)	31m 50s
almalinux-8 (almalinux:8)	9m 29s
linux-sha256 (ubuntu:rolling)	11m 48s
debian-11 (debian:11)	9m 35s
linux-meson (ubuntu:rolling)	12m 21s
linux-musl-meson (alpine:latest)	8m 5s
linux-TEST-vars (ubuntu:20.04)	13m 4s
linux-reftable-leaks (ubuntu:rolling)	34m 11s
linux-breaking-changes (ubuntu:rolling)	12m 14s
linux-reftable (ubuntu:rolling)	11m 45s
win+VS test	0s
win test (5)	5m 42s
win test (3)	3m 59s
win test (7)	5m 59s
win test (6)	5m 16s
win test (2)	5m 3s
win test (1)	7m 16s
win test (4)	7m 56s
win test (9)	6m 0s
win test (8)	7m 52s
win test (0)	6m 48s
win+Meson test (6)	8m 6s
win+Meson test (5)	6m 18s
win+Meson test (1)	8m 47s
win+Meson test (0)	7m 50s
win+Meson test (7)	6m 39s
win+Meson test (4)	10m 11s
win+Meson test (8)	9m 3s
win+Meson test (9)	8m 51s
win+Meson test (3)	7m 11s
win+Meson test (2)	6m 42s
	8h 51m 26s

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