fake_dns: fix fake IP allocation #426

Job	Run time
build-apple	17m 53s
build-android	5m 55s
build-bin-cross (x86_64-unknown-linux-musl)	4m 2s
build-bin-local (macos-latest)	10m 35s
build-bin-cross (i686-unknown-linux-musl)	3m 50s
test (macos-latest)	6m 3s
build-bin-cross (aarch64-unknown-linux-musl)	3m 53s
test (ubuntu-latest)	3m 30s
build-bin-cross (arm-unknown-linux-musleabi)	3m 59s
build-bin-cross (armv7-unknown-linux-musleabihf)	4m 0s
build-bin-cross (x86_64-pc-windows-gnu)	3m 59s
	1h 7m 39s

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