iostream: check that stream is open before trying to read (#2670)

Author: Zsailer

tornado/iostream.py

@@ -794,6 +794,7 @@ def _handle_read(self) -> None:
             self._read_from_buffer(pos)
 
     def _start_read(self) -> Future:
+        self._check_closed()  # Before reading, check that stream is not closed.
         assert self._read_future is None, "Already reading"
         self._read_future = Future()
         return self._read_future