bug#32880: 27.0.50; sieve-upload doesn't work if no sieve-buffer

[Eli Zaretskii]

> From: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
> Date: Sun, 30 Sep 2018 17:40:35 +0900
> 
> Found another issue while testing. `buffer-name' is used in `sieve-buffer'
> context too.
> 
> This adds the fix additionally by setting `script-name' before switching
> to `sieve-buffer'.

Thanks, pushed to the master branch.

In the future, please accompany the changes with a suitable commit log
message; see CONTRIBUTE for some guidance about that.

P.S. Would you like to write some tests for sieve.el?