Fix IssuerDN presence check in cert search result

When checking for presence of IssuerDN in certificate search result,
we mistakenly check for the presence of the SubjectDN field, then
unsafely index into the IssuerDN field.  Check the presence of
IssuerDN correctly.

Part of: https://fedorahosted.org/freeipa/ticket/4559

Reviewed-By: Martin Basti <mbasti@redhat.com>

ipaserver/plugins/dogtag.py

@@ -1903,7 +1903,7 @@ class ra(rabase.rabase):
                 response_request['subject'] = unicode(dn[0].text)
 
             issuer_dn = cert.xpath('IssuerDN')
-            if len(dn) == 1:
+            if len(issuer_dn) == 1:
                 response_request['issuer'] = unicode(issuer_dn[0].text)
 
             status = cert.xpath('Status')