use RITE_LV_NULL_MARK for better readability

The comment didn't say why the length is reserved.
Using RITE_LV_NULL_MARK makes things a bit clearer.

src/symbol.c

@@ -9,6 +9,7 @@
 #include "mruby.h"
 #include "mruby/khash.h"
 #include "mruby/string.h"
+#include "mruby/dump.h"
 
 /* ------------------------------------------------------ */
 typedef struct symbol_name {
@@ -38,7 +39,7 @@ KHASH_DEFINE (n2s, symbol_name, mrb_sym, TRUE, sym_hash_func, sym_hash_equal)
 static void
 sym_validate_len(mrb_state *mrb, size_t len)
 {
-  if (len > (UINT16_MAX-1)) {   /* UINT16_MAX is reverved */
+  if (len >= RITE_LV_NULL_MARK) {
     mrb_raise(mrb, E_ARGUMENT_ERROR, "symbol length too long");
   }
 }