pty.c: do nothing if terminated already

* ext/pty/pty.c (pty_detach_process): if the child process has
  terminated already, return immediately without starting a thread
  to detach it.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46129 b2dd03c8-39d4-4d8f-98ff-823fe69b080e