Coverity: fix issue in ipa_extdom_extop.c
    
    Coverity found the following issue:
    Error: BAD_COMPARE (CWE-697): [#def1]
    freeipa-4.6.5/daemons/ipa-slapi-plugins/ipa-extdom-extop/ipa_extdom_extop.c:121: null_misuse: Comparing pointer "threadnumber" against "NULL" using anything besides "==" or "!=" is likely to be incorrect.
    
    The comparison is using the pointer while it should use the pointed value.
    
    Fixes: https://pagure.io/freeipa/issue/7884
    Reviewed-By: Christian Heimes <cheimes@redhat.com>