Add new type predicate plistp

* lisp/subr.el (plistp): New type predicate (bug#47427).  This
referred to in the error message from plist-put: "Debugger
entered--Lisp error: (wrong-type-argument plistp (a b c))".

lisp/subr.el

@@ -4006,6 +4006,12 @@ Otherwise, return nil."
       (setq object (indirect-function object)))
   (and (subrp object) (eq (cdr (subr-arity object)) 'unevalled)))
 
+(defun plistp (object)
+  "Non-nil if and only if OBJECT is a valid plist."
+  (and (listp object)
+       (proper-list-p object)
+       (zerop (mod (length object) 2))))
+
 (defun macrop (object)
   "Non-nil if and only if OBJECT is a macro."
   (let ((def (indirect-function object)))

test/lisp/subr-tests.el

@@ -1081,5 +1081,14 @@ final or penultimate step during initialization."))
   (dolist (c (list ?a ?b ?α ?β))
     (should-not (char-uppercase-p c))))
 
+(ert-deftest test-plistp ()
+  (should (plistp nil))
+  (should-not (plistp 1))
+  (should (plistp '(1 2)))
+  (should-not (plistp '(1 . 2)))
+  (should (plistp '(1 2 3 4)))
+  (should-not (plistp '(1 2 3)))
+  (should-not (plistp '(1 2 3 . 4))))
+
 (provide 'subr-tests)
 ;;; subr-tests.el ends here