Fix heap buffer overflow when dump irep

Currently, the size of writing in heap by write_irep_record() is
bigger than The size that is calculated by get_irep_record_size.

Therefore, irep is dumped over the size of allocating memory when we
execute dump_irep().

src/dump.c

@@ -293,6 +293,7 @@ get_irep_record_size_1(mrb_state *mrb, const mrb_irep *irep)
   size_t size = 0;
 
   size += get_irep_header_size(mrb);
+  size += sizeof(uint16_t);
   size += get_iseq_block_size(mrb, irep);
   size += get_catch_table_block_size(mrb, irep);
   size += get_pool_block_size(mrb, irep);