Fix rx error with ? and ??

Fix rx error with ? and ??

The ? and ?? rx operators are special in that they can be written as
characters (space and '?' respectively).  This confused the definition
look-up mechanism in rare cases.

* lisp/emacs-lisp/rx.el (rx--expand-def): Don't look up non-symbols.
* test/lisp/emacs-lisp/rx-tests.el (rx-charset-or): Test.