EVP shake_ctrl(): add missing NULL evp_ctx check #1906

Job	Run time
basic_clang	7m 36s
basic_gcc	8m 45s
check_update	2m 1s
check-ansi	4m 38s
address_ub_sanitizer	16m 22s
check_docs	1m 12s
enable_brotli_dynamic	7m 16s
enable_non-default_options	9m 14s
enable_zstd_dynamic	7m 21s
fips_and_ktls	8m 9s
memory_sanitizer	22m 26s
minimal	3m 24s
non-caching	20m 32s
no-deprecated	9m 59s
no-shared (ubuntu-latest)	5m 40s
threads_sanitizer	6m 43s
buildtest	9m 57s
no-shared (macos-latest)	8m 5s
enable_brotli_and_asan_ubsan	13m 46s
enable_brotli_and_zstd_dynamic	7m 41s
enable_zstd_and_asan_ubsan	16m 3s
enable-tfo (ubuntu-latest)	8m 47s
external-test-cf-quiche	6m 36s
enable-tfo (macos-latest)	9m 33s
external-test-pyca (1.51.0, 3.9)	11m 46s
external-tests	14m 15s
no-legacy	7m 49s
legacy	5m 56s
out-of-source-and-install (ubuntu-latest)	10m 0s
out-of-source-and-install (macos-latest)	16m 6s
	4h 47m 38s

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