Fix an error for rake slow_cops

RuboCop will be inspected by a remote process when using `rubocop --server`.
As a result `RuboCop::Cop::Commissioner#trigger_responding_cops` is nothing:

```console
% rake prof:slow_cops
stackprof tmp/stackprof.dump --text --method
'RuboCop::Cop::Commissioner#trigger_responding_cops' rake aborted!
NoMethodError: undefined method `first' for nil:NilClass
tasks/prof.rake:28:in `block (2 levels) in <top (required)>'
Tasks: TOP => prof:slow_cops
```

This commit is inspected by a local process to prevent the above error.

tasks/prof.rake

@@ -5,6 +5,13 @@ namespace :prof do
 
   desc 'Run RuboCop on itself with profiling on'
   task :run, [:path] do |_task, args|
+    # Must be run `rubocop` with the local process.
+    require 'rubocop/server'
+    if RuboCop::Server.running?
+      RuboCop::Server::ClientCommand::Stop.new.run
+      puts 'Stop the server for profiling.'
+    end
+
     path = args.fetch(:path, '.')
     cmd = "bin/rubocop-profile #{path}"
     system cmd