Prevent errors if `destroy()` is called twice

Add checks before accessing object properties to see if they hadn't already been set to `null`.

Not modified the package `org.flixel.system.debug.*`

A few classes are destroyed automatically by Flixel (which guarantees they are only destroyed once), but some foolish user (or derailed bug) may decide to destroy something anyway, and since `if` checks are so cheap, it's better to be safe than sorry.