SPIGOT-6518: Anvils falling onto dripstone can sometimes crash server

By: md_5 <git@md-5.net>

paper-server/src/main/java/org/bukkit/craftbukkit/event/CraftEventFactory.java

@@ -876,7 +876,7 @@ public class CraftEventFactory {
         } else if (blockDamage != null) {
             DamageCause cause = null;
             Block damager = blockDamage;
-            if (source == DamageSource.CACTUS || source == DamageSource.SWEET_BERRY_BUSH || source == DamageSource.STALAGMITE) {
+            if (source == DamageSource.CACTUS || source == DamageSource.SWEET_BERRY_BUSH || source == DamageSource.STALAGMITE || source == DamageSource.ANVIL) {
                 cause = DamageCause.CONTACT;
             } else if (source == DamageSource.HOT_FLOOR) {
                 cause = DamageCause.HOT_FLOOR;