Land rapid7#4158, Fix for null dereference on Exploit::Remote::HttpServer#remove_resource

Author: jvazquez-r7

lib/msf/core/exploit/http/server.rb

@@ -547,7 +547,7 @@ def remove_resource(name)
     # Guard against removing resources added by other modules
     if @my_resources.include?(name)
       @my_resources.delete(name)
-      service.remove_resource(name)
+      service.remove_resource(name) if service
     end
   end