Use MRB_DUMP_OK instead of 0.

src/dump.c

@@ -689,7 +689,7 @@ mrb_write_irep(mrb_state *mrb, int top, char *bin)
 
   for (irep_no=top; irep_no<mrb->irep_len; irep_no++) {
     rc = write_irep_record(mrb, irep_no, bin, &rlen, DUMP_TYPE_BIN);
-    if (rc != 0)
+    if (rc != MRB_DUMP_OK)
       return rc;
 
     bin += (rlen + DUMP_SIZE(MRB_DUMP_SIZE_OF_LONG, DUMP_TYPE_BIN));
@@ -718,7 +718,7 @@ mrb_dump_irep(mrb_state *mrb, int top, FILE* fp)
 
   for (irep_no=top; irep_no<mrb->irep_len; irep_no++) {
     rc = dump_irep_record(mrb, irep_no, fp, &rlen);
-    if (rc != 0)
+    if (rc != MRB_DUMP_OK)
       return rc;
 
     rbds += rlen;