free NULL cleanup 9

Ongoing work to skip NULL check before calling free routine.  This gets:
    ecp_nistz256_pre_comp_free nistp224_pre_comp_free nistp256_pre_comp_free
    nistp521_pre_comp_free PKCS7_free PKCS7_RECIP_INFO_free
    PKCS7_SIGNER_INFO_free sk_PKCS7_pop_free PKCS8_PRIV_KEY_INFO_free
    PKCS12_free PKCS12_SAFEBAG_free PKCS12_free sk_PKCS12_SAFEBAG_pop_free
    SSL_CONF_CTX_free SSL_CTX_free SSL_SESSION_free SSL_free ssl_cert_free
    ssl_sess_cert_free

Reviewed-by: Kurt Roeckx <kurt@openssl.org>