bpo-41467: Fix asyncio recv_into() on Windows (pythonGH-21720)

On Windows, fix asyncio recv_into() return value when the socket/pipe
is closed (BrokenPipeError): return 0 rather than an empty byte
string (b'').
(cherry picked from commit 602a971)

Co-authored-by: Victor Stinner <vstinner@python.org>

Author: vstinner

Lib/asyncio/windows_events.py

@@ -469,7 +469,7 @@ def recv_into(self, conn, buf, flags=0):
             else:
                 ov.ReadFileInto(conn.fileno(), buf)
         except BrokenPipeError:
-            return self._result(b'')
+            return self._result(0)
 
         def finish_recv(trans, key, ov):
             try:

Misc/NEWS.d/next/Library/2020-08-04-00-20-30.bpo-41467.Z8DgTL.rst

@@ -0,0 +1,3 @@
+On Windows, fix asyncio ``recv_into()`` return value when the socket/pipe is
+closed (:exc:`BrokenPipeError`): return ``0`` rather than an empty byte
+string (``b''``).