worktree: Fix out of bounds read that causes data loss and reject invalid empty input in worktree add #29248

Job	Run time
config	6s
fuzz smoke test	2m 13s
win build	5m 16s
win+Meson build	4m 12s
sparse	3m 52s
documentation	7m 19s
rust-analysis	1m 42s
static-analysis	6m 38s
osx-clang (macos-14)	15m 28s
osx-reftable (macos-14)	13m 52s
osx-meson (macos-14)	14m 5s
osx-gcc (macos-14)	11m 48s
linux-sha256 (ubuntu:rolling)	15m 40s
linux-reftable (ubuntu:rolling)	15m 39s
linux32 (i386/ubuntu:20.04)	13m 39s
almalinux-8 (almalinux:8)	15m 17s
linux-breaking-changes (ubuntu:rolling)	15m 40s
linux-musl-meson (alpine:latest)	15m 36s
linux-asan-ubsan (ubuntu:rolling)	15m 37s
linux-reftable-leaks (ubuntu:rolling)	15m 38s
linux-leaks (ubuntu:rolling)	15m 37s
fedora-breaking-changes-meson (fedora:latest)	14m 30s
linux-TEST-vars (ubuntu:20.04)	15m 38s
win+VS build	0s
debian-11 (debian:11)	11m 53s
linux-meson (ubuntu:rolling)	15m 1s
win+VS test	0s
win+Meson test (2)	6m 16s
win+Meson test (1)	12m 53s
win+Meson test (7)	9m 56s
win+Meson test (4)	7m 33s
win+Meson test (5)	7m 57s
win+Meson test (8)	6m 30s
win+Meson test (3)	8m 10s
win+Meson test (9)	8m 46s
win+Meson test (0)	8m 56s
win+Meson test (6)	9m 2s
win test (0)	4m 16s
win test (2)	4m 12s
win test (4)	3m 59s
win test (7)	48s
win test (3)	14m 35s
win test (6)	3m 24s
win test (1)	14m 35s
win test (9)	1m 58s
win test (8)	14m 35s
win test (5)	14m 35s
	7h 24m 52s

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