Add delq list arg check

* src/fns.c (Fdelq): Check that list is a proper list.
This is more compatible with what ‘delete’ does.

src/fns.c

@@ -1589,6 +1589,7 @@ argument.  */)
       else
 	prev = tail;
     }
+  CHECK_LIST_END (tail, list);
   return list;
 }