worktree: Fix out of bounds read that causes data loss and reject invalid empty input in worktree add #29249

Job	Run time
config	6s
fuzz smoke test	1m 30s
win+Meson build	4m 12s
sparse	3m 33s
rust-analysis	2m 19s
win build	4m 12s
static-analysis	6m 45s
documentation	7m 35s
win+VS build	0s
almalinux-8 (almalinux:8)	16m 1s
osx-gcc (macos-14)	26m 8s
fedora-breaking-changes-meson (fedora:latest)	16m 34s
osx-reftable (macos-14)	24m 14s
linux-sha256 (ubuntu:rolling)	16m 39s
linux32 (i386/ubuntu:20.04)	13m 18s
osx-meson (macos-14)	32m 20s
osx-clang (macos-14)	26m 59s
debian-11 (debian:11)	15m 46s
linux-leaks (ubuntu:rolling)	56m 15s
linux-reftable (ubuntu:rolling)	19m 4s
linux-breaking-changes (ubuntu:rolling)	19m 2s
linux-reftable-leaks (ubuntu:rolling)	55m 7s
linux-musl-meson (alpine:latest)	16m 22s
linux-meson (ubuntu:rolling)	21m 33s
linux-asan-ubsan (ubuntu:rolling)	1h 5m 36s
linux-TEST-vars (ubuntu:20.04)	14m 49s
win+VS test	0s
win test (4)	6m 15s
win test (6)	10m 31s
win test (1)	9m 50s
win test (0)	10m 34s
win test (9)	16m 2s
win test (5)	9m 38s
win test (7)	8m 13s
win test (3)	8m 37s
win test (2)	8m 26s
win test (8)	7m 55s
win+Meson test (4)	7m 54s
win+Meson test (7)	9m 32s
win+Meson test (2)	9m 13s
win+Meson test (5)	8m 3s
win+Meson test (8)	7m 26s
win+Meson test (3)	10m 33s
win+Meson test (1)	16m 8s
win+Meson test (0)	13m 9s
win+Meson test (6)	7m 46s
win+Meson test (9)	11m 39s
	11h 23m 23s

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