FlxTilemap and FlxTilemapExt invalid frame checks

[Geokureli]

FlxTilemap and FlxTilemapExt should warn on invalid frames on creation rather than on collision/drawing, it should also replace the invalid tiles with 0
brought up in HaxeFlixel/flixel-addons#384

Edit: just remembered that checking for tiles < 0 are because many editors use -1 for empty tiles. keep that in mind when approaching this