Ensure TSPlayer.PlayerData is non-null whilst syncing loadouts

During connection, we receive a `SyncLoadout` packet before the
`ContinueConnecting2` packet, meaning we have not yet created a
`PlayerData` for this player.

TShockAPI/GetDataHandlers.cs

@@ -4445,6 +4445,11 @@ namespace TShockAPI
 				return true;
 			}
 
+			// Don't modify the player data if it isn't there.
+			// This is the case whilst the player is connecting, as we receive the SyncLoadout packet before the ContinueConnecting2 packet.
+			if (args.Player.PlayerData == null)
+				return false;
+
 			// The client does not sync slot changes when changing loadouts, it only tells the server the loadout index changed,
 			// and the server will replicate the changes the client did. This means that PlayerData.StoreSlot is never called, so we need to
 			// swap around the PlayerData items ourself.