fuzz/fuzz_rand.c: Add check for OSSL_LIB_CTX_new

fuzz/fuzz_rand.c: Add check for OSSL_LIB_CTX_new

As the potential failure of the OPENSSL_zalloc(), the OSSL_LIB_CTX_new()
could return NULL.
Therefore, it should be better to check it and return error if fails in
order to guarantee the success of the initialization.

Signed-off-by: Jiasheng Jiang <jiasheng@iscas.ac.cn>
Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/17738)