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.

lisp/emacs-lisp/rx.el

@@ -134,7 +134,7 @@ Each entry is:
                 (if (cdr def)
                     (error "Not an `rx' symbol definition: %s" form)
                   (car def)))))
-        ((consp form)
+        ((and (consp form) (symbolp (car form)))
          (let* ((op (car form))
                 (def (rx--lookup-def op)))
            (and def

test/lisp/emacs-lisp/rx-tests.el

@@ -316,7 +316,9 @@
   (should (equal (rx (not (or (in "abc") (char "bcd"))))
                  "[^a-d]"))
   (should (equal (rx (or (not (in "abc")) (not (char "bcd"))))
-                 "[^bc]")))
+                 "[^bc]"))
+  (should (equal (rx (or "x" (? "yz")))
+                 "x\\|\\(?:yz\\)?")))
 
 (ert-deftest rx-def-in-charset-or ()
   (rx-let ((a (any "badc"))