Handle null damage events for item frames. Fixes BUKKIT-5114

Some types of damage are handled specially so do not end up returning an
event in handleEntityDamageEvent. To ensure we don't have problems with
these we need to check for them and simply ignore them, as they've been
handled already.

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

@@ -24,10 +24,12 @@ public class EntityItemFrame extends EntityHanging {
         } else if (this.getItem() != null) {
             if (!this.world.isStatic) {
                 // CraftBukkit start
-                if (org.bukkit.craftbukkit.event.CraftEventFactory.handleEntityDamageEvent(this, damagesource, f).isCancelled() || this.dead) {
+                org.bukkit.event.entity.EntityDamageEvent event = org.bukkit.craftbukkit.event.CraftEventFactory.handleEntityDamageEvent(this, damagesource, f);
-                    return false;
+                if ((event != null && event.isCancelled()) || this.dead) {
+                    return true;
                 }
                 // CraftBukkit end
+
                 this.b(damagesource.getEntity(), false);
                 this.setItem((ItemStack) null);
             }