mitogen/parent: Bail out if STDIN or STDOUT is closed

Bail out if STDIN or STDOUT is closed/not available as it is used for the
communication with the parent process.

Signed-off-by: Marc Hartmayer <[email protected]>

Author: mhartmay

mitogen/parent.py

@@ -1429,6 +1429,8 @@ def __repr__(self):
     # "[1234 refs]" during exit.
     @staticmethod
     def _first_stage():
+        os.fstat(0)
+        os.fstat(1)
         R,W=os.pipe()
         r,w=os.pipe()
         if os.fork():