bug#70824: 29.3; Run time error in `dabbrev-completion'

[Matt Armstrong via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

Debugging this further (sorry, still no repro so I'm debugging my live 
image), I think this was introduced a few years ago in commit 
2d0085f756572856a2ed8d1bf043b59195a3e3f3 and partially fixed in commit 
d0975d7db03c231a3db5a1cd0edaf41094d43f0d.

Those commits introduced a feature where `dabbrev--find-expansion' 
considers the components of a buffer's file name as completion 
candidates. It does this by creating a temp buffer containing the 
components.

The problem is that this temp buffer is immediately killed, yet dabbrev 
variables still hold references to it. The second commit above correctly 
removes the killed buffer from `dabbrev--friend-buffer-list', but did 
not handle the case where `dabbrev--last-buffer' or 
`dabbrev--last-buffer-found' happened to be the just-killed buffer.

I can think of two approaches:

  - continual with the "manual GC" approach and scrub more dabbrev vars 
of this killed buffer.
  - handle the killed buffer gracefully by using buffer-live-p 
consistently, instead using checks against nil.