doveadm dump: Fix dumping "vsize" header in dovecot.index

It was trying to print it as the "vsize" header in dovecot.list.index.
This caused confusing output, as well as potentially accessing memory
out of bounds.

src/doveadm/doveadm-dump-index.c

@@ -715,7 +715,10 @@ static void dump_record(struct mail_index_view *view, unsigned int seq)
 			printf("                   : unseen   = %u\n", lrec->unseen);
 			printf("                   : recent   = %u\n", lrec->recent);
 			printf("                   : uidnext  = %u\n", lrec->uidnext);
-		} else if (strcmp(ext[i].name, "vsize") == 0) {
+		} else if (strcmp(ext[i].name, "vsize") == 0 &&
+			   ext[i].record_size >= sizeof(struct mailbox_index_vsize)) {
+			/* this is "vsize" in dovecot.list.index, not the
+			   32bit "vsize" in dovecot.index */
 			const struct mailbox_index_vsize *vrec = data;
 			printf("                   : vsize         = %"PRIu64"\n", vrec->vsize);
 			printf("                   : highest_uid   = %u\n", vrec->highest_uid);