lib-storage: Fix mailbox rename checking child mailbox name lengths

It was supposed to prevent allowing renames that would cause any child
mailbox name to be too long. However, the check wasn't working.

src/lib-storage/mail-storage.c

@@ -1755,15 +1755,16 @@ int mailbox_rename_check_children(struct mailbox *src, struct mailbox *dest)
 	size_t dest_prefix_len = strlen(dest->vname)+1;
 	/* this can return folders with * in their name, that are not
 	   actually our children */
-	const char *pattern = t_strdup_printf("%s%c*", src->vname,
-				  mail_namespace_get_sep(src->list->ns));
+	char ns_sep = mail_namespace_get_sep(src->list->ns);
+	const char *pattern = t_strdup_printf("%s%c*", src->vname, ns_sep);
 
 	struct mailbox_list_iterate_context *iter = mailbox_list_iter_init(src->list, pattern,
 				      MAILBOX_LIST_ITER_RETURN_NO_FLAGS);
 
 	const struct mailbox_info *child;
 	while((child = mailbox_list_iter_next(iter)) != NULL) {
-		if (strncmp(child->vname, src->vname, src_prefix_len) != 0)
+		if (strncmp(child->vname, src->vname, src_prefix_len-1) != 0 ||
+		    child->vname[src_prefix_len-1] != ns_sep)
 			continue; /* not our child */
 		/* if total length of new name exceeds the limit, fail */
 		if (strlen(child->vname + src_prefix_len)+dest_prefix_len > MAILBOX_LIST_NAME_MAX_LENGTH) {