vtable: fix UB detected as miri errors #13814

Job	Run time
android	1m 27s
build_and_test (ubuntu-22.04, stable)	2m 10s
cpp_test_driver (ubuntu-22.04)	2m 10s
build_and_test (ubuntu-22.04, 1.70)	2m 1s
cpp_cmake (ubuntu-22.04)	1m 57s
build_and_test (macos-11, stable)	2m 9s
cpp_test_driver (macos-12)	2m 9s
cpp_cmake (macos-12)	1m 58s
esp-idf	1m 57s
cpp_test_driver (windows-2022)	1m 56s
build_and_test (windows-2022, stable)	2m 4s
cpp_cmake (windows-2022)	1m 57s
build_and_test (windows-2022, 1.70)	2m 5s
ffi_32bit_build	1m 25s
build_and_test (windows-2022, beta, --exclude ffmpeg --exclude gstreamer-player)	2m 5s
fmt_test	1m 55s
mcu (pico-st7789, thumbv6m-none-eabi)	42s
mcu_esp	2m 8s
miri	11s
mcu (stm32h735g, thumbv7em-none-eabihf)	42s
node_test (ubuntu-22.04)	1m 5s
node_test (macos-11)	1m 3s
python_test (ubuntu-22.04)	20s
node_test (windows-2022)	56s
tree-sitter	1s
uefi-demo	1s
python_test (macos-11)	29s
python_test (windows-2022)	1s
updater_test (0.3.0)	1s
docs / docs	1s
wasm / wasm	1s
wasm_demo / wasm_demo	1s
wasm / build-slintpad	1s
wasm / lint-slintpad	1s
cpp_package_test	1s
	39m 11s

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