If we failed to unregister, don't fire the disconnect event

proxy/src/main/java/com/velocitypowered/proxy/VelocityServer.java

@@ -291,9 +291,10 @@ public class VelocityServer implements ProxyServer {
     return true;
   }
 
-  public void unregisterConnection(ConnectedPlayer connection) {
+  public boolean unregisterConnection(ConnectedPlayer connection) {
-    connectionsByName.remove(connection.getUsername().toLowerCase(Locale.US), connection);
+    boolean name = connectionsByName.remove(connection.getUsername().toLowerCase(Locale.US), connection);
-    connectionsByUuid.remove(connection.getUniqueId(), connection);
+    boolean uuid = connectionsByUuid.remove(connection.getUniqueId(), connection);
+    return name && uuid;
   }
 
   @Override

proxy/src/main/java/com/velocitypowered/proxy/connection/client/ConnectedPlayer.java

@@ -432,9 +432,10 @@ public class ConnectedPlayer implements MinecraftConnectionAssociation, Player {
     if (connectedServer != null) {
       connectedServer.disconnect();
     }
-    server.unregisterConnection(this);
+    if (server.unregisterConnection(this)) {
       server.getEventManager().fireAndForget(new DisconnectEvent(this));
     }
+  }
 
   @Override
   public String toString() {