Fix lightened nibble array getChunkSnapshot on extended block IDs

Fix out-of-bounds error in chunks with extended (4096) block IDs:

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

http://pastie.org/6368930

Thanks to mikeprimm for this fix.

Original MCPC+ commit:
MinecraftPortCentral/MCPC-Plus@7902767397

CraftBukkit-Patches/0009-Implement-lightening-of-NibbleArrays-only-allocate-b.patch

@@ -327,7 +327,7 @@ index 79c3893..97b7b5b 100644
 +                        if (tval != 0) {
 +                            tval = tval << 8;
 +                            for (int j = 0; j < 4096; j++) {
-+                                blockids[j << 1] |= tval;
++                                blockids[j] |= tval;
 +                            }
 +                        }
 +                  } else {