Fix getChunkSnapshot Spigot patch on extended block IDs

Fix out-of-bounds error at http://pastie.org/6368930

13:34:12 [SEVERE] [dynmap] Exception while fetching chunks:
java.lang.ArrayIndexOutOfBoundsException: 4096
    at org.bukkit.craftbukkit.v1_4_R1.CraftChunk.getChunkSnapshot(CraftChunk.java:1

Thanks to mikeprimm for suggesting this fix.