Fix an assignment to free variable warning

It fixes a bug introduced by commit
'query-replace-regexp undo: Update next-replacement after undo'
(30c4f35)

See https://lists.gnu.org/archive/html/emacs-devel/2019-09/msg00364.html
* lisp/replace.el(perform-replace): Rename variable
to next-replacement-replaced.

Author: calancha

lisp/replace.el

@@ -2711,7 +2711,7 @@ characters."
                                   search-string (nth (if replaced 4 3) elt)
                                   last-replacement (nth (if replaced 3 4) elt)
                                   search-string-replaced search-string
-                                  last-replacement-replaced last-replacement
+                                  next-replacement-replaced last-replacement
                                   last-was-act-and-show nil)
 
                                  (when (and (= stack-idx stack-len)