certmap: fix issue found by Coverity in OpenSSL version
    
    So far Coverity was only run with the NSS build enabled, with OpenSSL
    enabled an issue was found in an OpenSSL specific file.
    
    Related to https://pagure.io/SSSD/sssd/issue/3495
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>