examples: check array length before access (#21641)

Author: juan-db

examples/vweb_fullstack/src/auth_services.v

@@ -37,6 +37,9 @@ fn (mut app App) service_auth(username string, password string) !string {
 	users := sql db {
 		select from User where username == username
 	}!
+	if users.len == 0 {
+		return error('user not found')
+	}
 	user := users.first()
 	if user.username != username {
 		return error('user not found')