asyncio: Fix _ProactorWritePipeTransport._pipe_closed()

The "exc" variable was not defined, pass a BrokenPipeError exception instead.

Lib/asyncio/proactor_events.py

@@ -275,7 +275,7 @@ def _pipe_closed(self, fut):
         assert fut is self._read_fut, (fut, self._read_fut)
         self._read_fut = None
         if self._write_fut is not None:
-            self._force_close(exc)
+            self._force_close(BrokenPipeError())
         else:
             self.close()