Set server variable on GET /action/image/update

Otherwise, ogCP crashes on image update.

Fixes: 8726ade ("Adapt commands to work with several ogServers")

ogcp/views.py

@@ -1189,6 +1189,7 @@ def action_image_update():
         return redirect(url_for('commands'))
     form.ip.data = ' '.join(ips)
 
+    server = get_server_from_clients(ips)
     r = server.get('/images')
     for image in r.json()['images']:
         form.image.choices.append((image['id'], image['name']))