[C] Check if player is still connected after event fired. Fixes BUKKIT-4327

#### The Issue

When trying to kick a player on a respawn event, the server will send a packet disconnecting the client, but keep the player in the server permanently.   

#### Justification for PR

This bug can be crippling for the server as the phantom player takes up a slot, and keeps the player's name. It also prevents the admin from cleanly stopping the server.

#### PR Breakdown

After firing the ```PlayerRespawnEvent``` it will check if the player is still connected. If the player is no longer connected, the method will return immediately. 

#### JIRA Ticket

BUKKIT-4327 - https://bukkit.atlassian.net/browse/BUKKIT-4327