rtt: initialize min_rtt to zero #5120

Job	Run time
quiche_macos (macos-13)	3m 32s
quiche	3m 45s
quiche_windows (x86_64-pc-windows-msvc)	4m 16s
quiche_macos (macos-latest)	2m 31s
quiche (boringssl-boring-crate)	3m 21s
quiche_windows (i686-pc-windows-msvc)	3m 37s
quiche (openssl)	6m 9s
quiche_windows (x86_64-pc-windows-gnu)	6m 13s
quiche_windows (i686-pc-windows-gnu)	7m 9s
android_ndk_lts (aarch64-linux-android)	1m 30s
android_ndk_lts (armv7-linux-androideabi)	1m 39s
docker	3m 11s
android_ndk_lts (x86_64-linux-android)	1m 30s
http3_test	52s
android_ndk_lts (i686-linux-android)	1m 26s
nginx (1.16.1)	57s
quiche_ios (x86_64-apple-ios)	1m 27s
quiche_multiarch (aarch64-unknown-linux-gnu)	2m 56s
quiche_ios (aarch64-apple-ios)	1m 13s
quiche_multiarch (armv7-unknown-linux-gnueabihf)	3m 27s
quiche_multiarch (i686-unknown-linux-gnu)	1m 54s
	1h 2m 35s

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