Fix lightened nibble array getChunkSnapshot on extended block IDs

Fix out-of-bounds error in chunks with extended (4096) block IDs.

Not that this will occur on 'vanilla' Spigot servers.. but might as well submit it back upstream ;). We have had this change it in MCPC+ for a while now:

MinecraftPortCentral/MCPC-Plus@790276739766cb1733cc0787aaeb88ae6a507608

and its been working well.