LDAP: Fix Dereference after NULL check
    
    The commit dc30c60f166ad9adc63a47a1013508a71624ac87
    changed the logic in NULL check
     -    if (protocol) {
     +    if (protocol == NULL) {
    
    Found by Coverity:
    
    Reviewed-by: Petr Čech <pcech@redhat.com>