certdb: fix AttributeError in verify_ca_cert_validity

`NSSDatabase.verify_ca_cert_validity` tries to access a property of basic
constraints extension on the extension object itself rather than its value.

Access the attribute on the correct object to fix the issue.

Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>

ipapython/certdb.py

@@ -569,7 +569,7 @@ class NSSDatabase(object):
         except cryptography.x509.ExtensionNotFound:
             raise ValueError("missing basic constraints")
 
-        if not bc.ca:
+        if not bc.value.ca:
             raise ValueError("not a CA certificate")
 
         try: