magit-stash-drop: abort if multi-stash deletion is not confirmed

Fixes #2917.

Documentation/RelNotes/2.10.1.txt

@@ -4,5 +4,9 @@ Magit v2.10.1 Release Notes
 Fixes since v2.10.0
 -------------------
 
+* When multiple stashes were selected by the region, responding "no"
+  when asked to confirm the deletion of these stashes resulted in a
+  type error.  #2917
+
 Authors
 -------

lisp/magit-stash.el

@@ -214,7 +214,8 @@ and forgo removing the stash."
   "Remove a stash from the stash list.
 When the region is active offer to drop all contained stashes."
   (interactive (list (--if-let (magit-region-values 'stash)
-                         (magit-confirm t nil "Drop %i stashes" it)
+                         (or (magit-confirm t nil "Drop %i stashes" it)
+                             (user-error "Abort"))
                        (magit-read-stash "Drop stash"))))
   (dolist (stash (if (listp stash)
                      (nreverse (prog1 stash (setq stash (car stash))))