Cygwin: pipe: Fix raw_write() for non-cygwin pipe with size zero.

- Currently, raw_write() fails to handle size zero pipe which may
  be created by non-cygwin apps (e.g. Windows native ninja). This
  patch fixes the issue.

Addresses:
  https://cygwin.com/pipermail/cygwin/2021-November/249844.html

winsup/cygwin/fhandler_pipe.cc

@@ -449,7 +449,7 @@ fhandler_pipe_fifo::raw_write (const void *ptr, size_t len)
       return -1;
     }
 
-  if (len <= pipe_buf_size)
+  if (len <= pipe_buf_size || pipe_buf_size == 0)
     chunk = len;
   else if (is_nonblocking ())
     chunk = len = pipe_buf_size;