ossl-nghttp3-demo-server.c: Fix compatibility with various clients #407

Job	Run time
address_ub_sanitizer	1s
basic_clang	1s
basic_gcc	1s
check_docs	1s
check_update	1s
check-ansi	1s
enable_non-default_options	1s
external-test-cf-quiche	1s
external-test-pyca (1.51.0, 3.9)	1s
external-tests	1s
fuzz_tests	1s
full_featured	1s
legacy	1s
memory_sanitizer	1s
minimal	1s
no-legacy	1s
no-deprecated	1s
non-caching	1s
no-shared-macos (macos-13)	9m 23s
no-shared-ubuntu	1s
no-shared-macos (macos-14)	4m 13s
out-of-readonly-source-and-install-macos (macos-13)	11m 44s
out-of-readonly-source-and-install-ubuntu	1s
self-hosted	0s
out-of-readonly-source-and-install-macos (macos-14)	6m 40s
threads_sanitizer	1s
	32m 21s

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