bug#28013: 26.0.50; Gnus signals error on deleting attachment

[Stephen Berman]

I typed `d' in the Gnus *Article* buffer on the button of the attachment
to a mail delete to delete the attachment, and while the deletion
succeeded, the following error was signalled:

Debugger entered--Lisp error: (error "No such part")
  signal(error ("No such part"))
  error("No such part")
  gnus-article-jump-to-part(0)
  gnus-mime-delete-part()
  funcall-interactively(gnus-mime-delete-part)
  call-interactively(gnus-mime-delete-part nil nil)
  command-execute(gnus-mime-delete-part)

This occurred because the message body consisted only of the attachment.
The mail was in mbox format and the mail back end was nnfolder.

In GNU Emacs 26.0.50 (build 3, x86_64-pc-linux-gnu, GTK+ Version 3.22.8)
 of 2017-08-08 built on rosalinde
Repository revision: 846870e508021ee8d1099280b3f40fe108a34bf0