fix ubsan vvfat #660

Job	Run time
checkapply	9s
build-clang	1m 48s
build	1m 48s
build-big-endian	1m 49s
build-32bits	1m 49s
build-clang-latest	1m 49s
build-cross-mingw64	1m 45s
build-debug	1m 46s
build-cross (alpine)	1m 48s
build-disable-kvm	1m 46s
build-cross (centos9)	1m 48s
build-disable-tcg	1m 44s
build-disable-tcg-kvm-for-xen	1m 46s
build-cross (debian)	1m 48s
build-macos-aarch64	1m 47s
build-cross (debian-all-test-cross)	1m 48s
build-macos-x86_64	1m 44s
build-cross (debian-amd64-cross)	1m 48s
build-minimal	1m 45s
build-cross (debian-arm64-cross)	1m 46s
build-misc	1m 46s
build-cross (debian-armhf-cross)	1m 48s
build-rust	1m 46s
build-static	1m 44s
build-cross (debian-hexagon-cross)	1m 48s
build-tsan	1s
build-cross (debian-i686-cross)	1m 46s
build-windows (UCRT64)	1s
check	1s
build-cross (debian-legacy-test-cross)	1m 47s
build-windows (CLANG64)	1s
check-avocado	1s
build-windows (MINGW64)	1s
build-cross (debian-loongarch-cross)	1m 47s
check-block	1s
build-cross (debian-mips64el-cross)	1m 46s
checkpatch-ignore-signoff	1s
build-cross (debian-mipsel-cross)	1m 46s
checkpatch-with-signoff	1s
check-tcg	1s
build-cross (debian-ppc64el-cross)	1m 46s
build-cross (debian-riscv64-cross)	1m 45s
build-cross (debian-s390x-cross)	1m 45s
build-cross (debian-tricore-cross)	1m 47s
build-cross (fedora)	1m 44s
build-cross (fedora-rust-nightly)	1m 45s
build-cross (opensuse-leap)	1m 46s
build-cross (ubuntu2204)	1m 46s
	1h 5m 59s

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