vm.c, codedump.c: add `IREP_TT_NFLAG` assertions.

The pool specified by `OP_STRING` (and `OP_SYMBOL`) should represent a
string, so that `IREP_TT_NFLAG` should be zero.

src/codedump.c

@@ -461,6 +461,7 @@ codedump(mrb_state *mrb, const mrb_irep *irep)
       print_lv_a(mrb, irep, a);
       break;
     CASE(OP_STRING, BB):
+      mrb_assert((irep->pool[b].tt&IREP_TT_NFLAG)==0);
       if ((irep->pool[b].tt & IREP_TT_NFLAG) == 0) {
         printf("OP_STRING\tR%d\tL(%d)\t; %s", a, b, irep->pool[b].u.str);
       }

src/vm.c

@@ -2652,6 +2652,7 @@ RETRY_TRY_BLOCK:
     CASE(OP_STRING, BB) {
       size_t len;
 
+      mrb_assert((pool[b].tt&IREP_TT_NFLAG)==0);
       len = pool[b].tt >> 2;
       if (pool[b].tt & IREP_TT_SFLAG) {
         regs[a] = mrb_str_new_static(mrb, pool[b].u.str, len);