allow NULL (no variable) in lvar section of mrb format; fix #2294

This fix use UINT16_MAX for NULL symbol tag, that means maximum symbol length
is not UINT16_MAX-1.

include/mruby/dump.h

@@ -94,6 +94,8 @@ struct rite_section_lv_header {
   RITE_SECTION_HEADER;
 };
 
+#define RITE_LV_NULL_MARK              UINT16_MAX
+
 struct rite_binary_footer {
   RITE_SECTION_HEADER;
 };