Fix TypeError when the system lacks memory

This fixes up commit e3781aff
(Reduce probability of dying when the system lacks memory").

re6st/utils.py

@@ -156,6 +156,7 @@ class Popen(subprocess.Popen):
             self.returncode = -1
 
     def stop(self):
+        if self.pid:
             self.terminate()
             t = threading.Timer(5, self.kill)
             t.start()