bug#20538: 24.4; Attempt to delete a surrogate minibuffer frame

[Drew Adams]

> emacs -Q foo bar
> M-x ediff-buffers ; to compare buffers foo and bar
> ; select buffer foo
> 
> C-x 5 2 ; display buffer foo in a 2nd window in new frame

I was going to guess that it had something to do with having
one of the compared buffers in a separate frame, i.e., two
frames showing a buffer being compared.

That's been problematic all along, AFAIK.  It can make Ediff
tremble in the knees.  Especially if you do something like this:

> M-x delete-frame

That kind of thing can also result in Ediff screaming that
you've killed one of its relatives.