Check connection status before setting scoreboard. Fixes BUKKIT-4064

Two connection status checks were added to setting a scoreboard for a
player. The first checks to see if a player has logged in yet, which
implicates the ability to receive packets. The second checks to affirm
that the CraftPlayer reference is still to a logged in player; setting
it while not logged in would maintain a stale player reference in the
scoreboard manager.

src/main/java/org/bukkit/craftbukkit/entity/CraftPlayer.java

@@ -983,6 +983,14 @@ public class CraftPlayer extends CraftHumanEntity implements Player {
 
     public void setScoreboard(Scoreboard scoreboard) {
         Validate.notNull(scoreboard, "Scoreboard cannot be null");
+        PlayerConnection playerConnection = getHandle().playerConnection;
+        if (playerConnection == null) {
+            throw new IllegalStateException("Cannot set scoreboard yet");
+        }
+        if (playerConnection.disconnected) {
+            throw new IllegalStateException("Cannot set scoreboard for invalid CraftPlayer");
+        }
+
         this.server.getScoreboardManager().setPlayerBoard(this, scoreboard);
     }
 }