doveadm direcor add: Don't accept number as a valid hostname.

This is usually a mistake. For example "doveadm director add -t host 100"
shouldn't be adding a new IP 100 (0.0.0.100)

src/doveadm/doveadm-director.c

@@ -469,6 +469,13 @@ cmd_director_add_or_update(struct doveadm_cmd_context *cctx, bool update)
 		director_cmd_help(cctx->cmd);
 		return;
 	}
+	if (str_to_uint(host, &i) == 0) {
+		/* host is a number. this would translate to an IP address,
+		   which is probably a mistake. */
+		i_error("Invalid host '%s'", host);
+		director_cmd_help(cctx->cmd);
+		return;
+	}
 
 	host = ctx->host;
 	if (ctx->tag == NULL) {