bug#23536: 25.1.50; viper-mode "/" search includes "(default ...) " prefix

[Jim Meyering]

This is using emacs commit (latest master)
c8109d9c4057d8cac79e2c139758cadd410e7446.
How to repro?
Invoke this: emacs -nw -Q -f viper-mode
Then answer "n" to the viperize prompt.
Type "/foo" and hit ENTER.
You'll notice that the minibuffer now contains this:

  ‘(default ) foo’: Pattern not found

implying that you've searched for a string significantly different than "foo".
If you try again, you'll see that the search string grows even longer:

  ‘(default (default ) foo) foo’: Pattern not found

It looks like this problem was introduced via commit
088acab3831b45e0e0749705226b8680076df4b6, since
if I revert that and rebuild, the problem goes away.