Fix off-by-one-link error in image--set-property

* lisp/image.el (image--set-property): Ensure new value is set even
in the unlikely case that the plist is empty.  Fix off-by-one-link
error when deleting a property. (bug#35285)
* test/lisp/image-tests.el: New file.
(image--set-property): New test.

lisp/image.el

@@ -452,10 +452,10 @@ Internal use only."
         ;; plist.  Decouple plist entries where the key matches
         ;; the property.
         (if (eq (cadr image) property)
-            (setcdr image (cddr image))
+            (setcdr image (cdddr image))
           (setq image (cddr image))))
     ;; Just enter the new value.
-    (plist-put (cdr image) property value))
+    (setcdr image (plist-put (cdr image) property value)))
   value)
 
 (defun image-property (image property)

test/lisp/image-tests.el

@@ -0,0 +1,45 @@
+;;; image-tests.el --- tests for image.el -*- lexical-binding: t -*-
+
+;; Copyright (C) 2019 Free Software Foundation, Inc.
+
+;; This file is part of GNU Emacs.
+
+;; GNU Emacs is free software: you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; GNU Emacs is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.
+
+;;; Code:
+
+(require 'ert)
+(require 'image)
+
+(ert-deftest image--set-property ()
+  "Test `image--set-property' behavior."
+  (let ((image (list 'image)))
+    ;; Add properties.
+    (setf (image-property image :scale) 1)
+    (should (equal image '(image :scale 1)))
+    (setf (image-property image :width) 8)
+    (should (equal image '(image :scale 1 :width 8)))
+    (setf (image-property image :height) 16)
+    (should (equal image '(image :scale 1 :width 8 :height 16)))
+    ;; Delete properties.
+    (setf (image-property image :type) nil)
+    (should (equal image '(image :scale 1 :width 8 :height 16)))
+    (setf (image-property image :scale) nil)
+    (should (equal image '(image :width 8 :height 16)))
+    (setf (image-property image :height) nil)
+    (should (equal image '(image :width 8)))
+    (setf (image-property image :width) nil)
+    (should (equal image '(image)))))
+
+;;; image-tests.el ends here