cmp_kem.c: fix return in case of error #1876

Job	Run time
address_ub_sanitizer	15m 38s
basic_clang	9m 51s
basic_gcc	8m 1s
buildtest	11m 36s
check_docs	58s
check_update	1m 41s
enable_brotli_and_asan_ubsan	13m 38s
check-ansi	4m 48s
enable_brotli_and_zstd_dynamic	7m 17s
enable_brotli_dynamic	9m 35s
enable_non-default_options	8m 48s
enable_zstd_and_asan_ubsan	14m 41s
enable_zstd_dynamic	7m 4s
enable-tfo (ubuntu-latest)	8m 24s
external-test-cf-quiche	6m 44s
enable-tfo (macos-latest)	11m 22s
external-test-pyca (1.51.0, 3.9)	11m 21s
legacy	7m 17s
external-tests	14m 26s
fips_and_ktls	7m 46s
memory_sanitizer	20m 47s
no-deprecated	8m 44s
minimal	3m 42s
no-legacy	7m 53s
non-caching	19m 16s
no-shared (ubuntu-latest)	5m 35s
out-of-source-and-install (ubuntu-latest)	9m 55s
no-shared (macos-latest)	8m 26s
threads_sanitizer	6m 24s
out-of-source-and-install (macos-latest)	17m 2s
	4h 48m 40s

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