check if running as system

Author: NickTyrer

modules/exploits/windows/local/wmi_persistence.rb

@@ -91,6 +91,11 @@ def exploit
       return
    end
 
+   if is_system?
+      print_error("This module cannot run as System")
+      return
+   end
+
    host = session.session_host
    print_status('Installing Persistence...')