lib: fix potential underflow in RangeSet Debug impl #5028

Job	Run time
android_ndk_lts (aarch64-linux-android)	1m 30s
docker	4m 7s
android_ndk_lts (armv7-linux-androideabi)	1m 33s
http3_test	56s
android_ndk_lts (x86_64-linux-android)	1m 31s
android_ndk_lts (i686-linux-android)	1m 36s
nginx (1.16.1)	1m 13s
quiche	3m 59s
quiche (boringssl-boring-crate)	3m 31s
quiche_ios (x86_64-apple-ios)	1m 56s
quiche (openssl)	6m 15s
quiche_macos (macos-latest)	1m 51s
quiche_ios (aarch64-apple-ios)	1m 20s
quiche_macos (macos-14)	2m 17s
quiche_multiarch (aarch64-unknown-linux-gnu)	3m 34s
quiche_windows (x86_64-pc-windows-msvc)	3m 34s
quiche_multiarch (armv7-unknown-linux-gnueabihf)	3m 40s
quiche_windows (i686-pc-windows-msvc)	3m 56s
quiche_multiarch (i686-unknown-linux-gnu)	2m 46s
quiche_windows (x86_64-pc-windows-gnu)	5m 16s
quiche_windows (i686-pc-windows-gnu)	4m 40s
	1h 1m 1s

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