Don't try to pretty-print non-lists as binding list

* lisp/emacs-lisp/pp.el (pp--format-definition): Ensure that what we try
to print as a list of bindings has an appropriate format.  This avoids
raising an error for SEXPs like (let X Y) inside `pcase' forms where our
heuristic expects a binding list in the X position.

lisp/emacs-lisp/pp.el

@@ -577,7 +577,8 @@ the bounds of a region containing Lisp code to pretty-print."
     (unless (consp edebug)
       (setq edebug nil))
     (if (and (consp (car edebug))
-             (eq (caar edebug) '&rest))
+             (eq (caar edebug) '&rest)
+             (proper-list-p (car sexp)))
         (pp--insert-binding (pop sexp))
       (if (null (car sexp))
           (insert "()")