bug#34974: 27.0.50; Moving article error with duplicate suppression disabled

[Basil L. Contovounesios]

With gnus-suppress-duplicates left at its default value of nil, trying
to move an article with 'B m <group>' gives me the following backtrace:

Debugger entered--Lisp error: (wrong-type-argument hash-table-p nil)
  remhash("<redacted-message-id>" nil)
  gnus-dup-unsuppress-article(1988)
  gnus-summary-move-article(nil)
  funcall-interactively(gnus-summary-move-article nil)
  call-interactively(gnus-summary-move-article nil nil)
  command-execute(gnus-summary-move-article)

I'm no expert, and I haven't tried reproducing this with a minimal
config, but I think gnus-summary-move-article should not call
gnus-dup-unsuppress-article when gnus-suppress-duplicates is nil, right?

This issue seems to have been uncovered by the switch to hash-tables in
bug#33653.  Previously, gnus-dup-unsuppress-article called unintern,
which would not complain when its second argument gnus-dup-hashtb was
nil, even though it probably should have.

Patch to follow.

Thanks,

-- 
Basil

Gnus v5.13
In GNU Emacs 27.0.50 (build 1, x86_64-pc-linux-gnu, X toolkit, Xaw3d scroll 
bars)
 of 2019-03-24 built on thunk
Repository revision: dbd6490ad49b0f088d56cdd5f04178bdd62c806a
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12003000
System Description: Debian GNU/Linux buster/sid