P11: Don't return int failure from a bool function
    
    The functions return bool as per their prototype, but returning EINVAL
    on failure meant that EINVAL (typically 22) was converted to 'true', so
    a certificate that was not processable was considered valid.
    
    Luckily this code only converts certificates into SSH public keys, so
    there are no security implications.
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>
    Reviewed-by: Sumit Bose <sbose@redhat.com>