Don't panic on empty header block #5381

Job	Run time
quiche_macos (macos-13)	3m 42s
quiche_macos (macos-latest)	1m 54s
http3_test	46s
quiche	4m 33s
quiche_windows (x86_64-pc-windows-msvc)	5m 13s
quiche (boringssl-boring-crate)	4m 30s
quiche_ios (x86_64-apple-ios)	1m 17s
quiche_multiarch (aarch64-unknown-linux-gnu)	2m 58s
quiche (openssl)	7m 7s
quiche_ios (aarch64-apple-ios)	1m 34s
quiche_windows (i686-pc-windows-msvc)	4m 13s
quiche_multiarch (armv7-unknown-linux-gnueabihf)	3m 17s
quiche_windows (x86_64-pc-windows-gnu)	5m 55s
quiche_multiarch (i686-unknown-linux-gnu)	1m 43s
quiche_windows (i686-pc-windows-gnu)	7m 51s
android_ndk_lts (aarch64-linux-android)	1m 0s
docker	2m 4s
nginx (1.16.1)	52s
android_ndk_lts (armv7-linux-androideabi)	1m 15s
android_ndk_lts (x86_64-linux-android)	47s
android_ndk_lts (i686-linux-android)	1m 8s
	1h 3m 39s

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