Fix crash with null thorns damager

nms-patches/EnchantmentThorns.patch

@@ -0,0 +1,11 @@
+--- ../work/decompile-8eb82bde/net/minecraft/server/EnchantmentThorns.java	2014-12-10 15:53:01.980344940 +1100
++++ src/main/java/net/minecraft/server/EnchantmentThorns.java	2014-12-10 15:52:50.688364236 +1100
+@@ -29,7 +29,7 @@
+         Random random = entityliving.bb();
+         ItemStack itemstack = EnchantmentManager.a(Enchantment.THORNS, entityliving);
+ 
+-        if (a(i, random)) {
++        if (entity != null && a(i, random)) { // CraftBukkit
+             entity.damageEntity(DamageSource.a(entityliving), (float) b(i, random));
+             entity.makeSound("damage.thorns", 0.5F, 1.0F);
+             if (itemstack != null) {