event attach does not check for error

[whatvn]

https://github.com/wandenberg/redis_nginx_adapter/blob/f3f3cbd47b2c63d72cc315d3d2acf8ae6bfb2c99/redis_nginx_adapter.c#L75

redis_nginx_event_attach won't add event data into current context if there's error. This could cause error later if we use context. Should we just clean up data and return error instead of keep going likes the current way?