Fix Redex variable-not-otherwise-mentioned bug.

And add a test to keep it from happening again.

pkgs/redex-pkgs/redex-lib/redex/private/recursive-lang.rkt

@@ -242,7 +242,9 @@
                      [else (loop sub-pat)]))
                 sub-pats)]
               [(? (compose not pair?))
-               (set pat)])))
+               (if (symbol? pat)
+                   (set pat)
+                   (set))])))
         (nt-rhs the-nt)))
     lang)))
 

pkgs/redex-pkgs/redex-test/redex/tests/enum-test.rkt

@@ -80,7 +80,7 @@
 
 ;; test variable-not-otherwise-mentioned
 (define-language VarMentioned
-  (mention a b c x y z)
+  (mention a b c x y z 2 #f #\c (vec 1 2))
   (var variable-not-otherwise-mentioned))
 
 (try-it 20 VarMentioned var)