doveadm sync/backup: Don't override BROKENCHAR if it's already set

This allows migrating invalid mailbox names by specifying BROKENCHAR.
Previously it would always try to use \003 control character, which isn't
valid character in mailbox names so the mailbox creation would fail.

src/doveadm/doveadm-dsync.c

@@ -326,8 +326,10 @@ static void doveadm_user_init_dsync(struct mail_user *user)
 	struct mail_namespace *ns;
 
 	user->dsyncing = TRUE;
-	for (ns = user->namespaces; ns != NULL; ns = ns->next)
-		ns->list->set.broken_char = DSYNC_LIST_BROKEN_CHAR;
+	for (ns = user->namespaces; ns != NULL; ns = ns->next) {
+		if (ns->list->set.broken_char == '\0')
+			ns->list->set.broken_char = DSYNC_LIST_BROKEN_CHAR;
+	}
 }
 
 static bool paths_are_equal(struct mail_user *user1, struct mail_user *user2,