Loading the following file triggers an assertion failure:

;;; -*- lexical-binding: t; -*-
(defvar 1)

$ emacs -Q -batch -l /tmp/crash.el
eval.c:772: Emacs fatal error: assertion failed: SYMBOLP (sym)
Fatal error 6: Abort trapAbort trap: 6

This is because Fdefvar lacks a CHECK_SYMBOL in the second branch
(lexical binding but only one argument)