fix only_v6() panic on windows #1340

Job	Run time
Rustfmt	15s
Test (beta)	21s
Test (stable)	21s
Test (nightly)	20s
Test (macos)	17s
Test (1.70.0)	25s
Clippy	15s
Test (windows)	58s
check-external-types (ubuntu-latest)	21s
check-external-types (macos-latest)	19s
check-external-types (windows-latest)	39s
Check (aarch64-apple-darwin)	35s
Check (aarch64-apple-ios)	38s
Check (aarch64-apple-tvos)	31s
Check (aarch64-unknown-linux-musl)	33s
Check (aarch64-apple-watchos)	32s
Check (aarch64-unknown-freebsd)	38s
Check (aarch64-unknown-linux-gnu)	34s
Check (aarch64-linux-android)	33s
Check (aarch64-unknown-openbsd)	32s
Check (aarch64-unknown-linux-ohos)	33s
Check (arm-linux-androideabi)	34s
Check (armv7-linux-androideabi)	31s
Check (i686-linux-android)	32s
Check (armv7-sony-vita-newlibeabihf)	33s
Check (aarch64-unknown-netbsd)	35s
Check (x86_64-pc-solaris)	35s
Check (x86_64-apple-darwin)	34s
Check (sparcv9-sun-solaris)	34s
Check (x86_64-unknown-freebsd)	34s
Check (armv7-unknown-linux-ohos)	32s
Check (aarch64-unknown-redox)	33s
Check (arm64_32-apple-watchos)	32s
Check (x86_64-pc-windows-msvc)	30s
Check (x86_64-unknown-illumos)	34s
Check (x86_64-apple-ios)	35s
Check (x86_64-unknown-linux-musl)	35s
Check (x86_64-pc-cygwin)	32s
Check (x86_64-unknown-linux-gnu)	33s
Check (i686-unknown-linux-gnu)	35s
Check (x86_64-unknown-dragonfly)	31s
Check (x86_64-unknown-linux-ohos)	35s
Check (x86_64-unknown-openbsd)	33s
Check (x86_64-unknown-fuchsia)	34s
Check (x86_64-unknown-netbsd)	31s
Check (x86_64-unknown-redox)	35s
	24m 2s

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