win,pipe: fix another missing assignment to success (#4523)

Yet another followup to #4511. The functional/legacy/increment_spec.lua
test failed most of the time without this, and passes consistently with
it. It seemed unexpected this code path gets reached (perhaps imply
that the user wrote zero bytes?), but good to fix of course.

src/win/pipe.c

@@ -1983,6 +1983,7 @@ static int uv__pipe_read_data(uv_loop_t* loop,
     } else {
       if (max_bytes > bytes_available)
         max_bytes = bytes_available;
+      *bytes_read = 0;
       if (max_bytes == 0 || ReadFile(handle->handle, buf.base, max_bytes, bytes_read, NULL))
         r = ERROR_SUCCESS;
       else