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

ext/pty/pty.c

@@ -538,6 +538,11 @@ pty_open(VALUE klass)
 static VALUE
 pty_detach_process(struct pty_info *info)
 {
+#ifdef WNOHANG
+    int st;
+    if (rb_waitpid(info->child_pid, &st, WNOHANG) <= 0)
+	return Qnil;
+#endif
     rb_detach_process(info->child_pid);
     return Qnil;
 }