Check against python None type when filling in auth parameters

When deciding whether to provide an auth function callback
in openAuth(), credcb was checked against NULL, when it
really needs to be checked against Py_None
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

python/libvirt-override.c

@@ -1840,7 +1840,8 @@ libvirt_virConnectOpenAuth(PyObject *self ATTRIBUTE_UNUSED, PyObject *args) {
             auth.credtype[i] = (int)PyLong_AsLong(val);
         }
     }
-    auth.cb = pycredcb ? virConnectCredCallbackWrapper : NULL;
+    if (pycredcb && pycredcb != Py_None)
+        auth.cb = virConnectCredCallbackWrapper;
     auth.cbdata = pyauth;
 
     LIBVIRT_BEGIN_ALLOW_THREADS;