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

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.

(cherry picked from commit 3580dc155c3c9f48fb1b7855b4d858eec3948dfb)