Return the valid max value for piston direction. Fixes BUKKIT-2191

By returning the following value (7) we remove the need to special
case pistons in any way (other than the original purpose of this
check, which is to ensure pistons have valid data)

src/main/java/net/minecraft/server/BlockPiston.java

@@ -217,7 +217,7 @@ public class BlockPiston extends Block {
     }
 
     public static int e(int i) {
-        if ((i & 7) >= Facing.OPPOSITE_FACING.length) return 0; // CraftBukkit - check for AIOOB on piston data
+        if ((i & 7) >= Facing.OPPOSITE_FACING.length) return 7; // CraftBukkit - check for AIOOB on piston data
         return i & 7;
     }