Ensure entity is always dying before Death Event

Prior to this it was possible for plugins to put the server into a never
ending recursive loop until it eventually killed itself.

Fixes GH-1432

.gitignore

@@ -34,6 +34,7 @@ work/Spigot-Server
 work/Spigot-API
 work/*.jar
 work/test-server
+work/ForgeFlower
 
 # Mac filesystem dust
 .DS_Store/

Spigot-Server-Patches/Improve-death-events.patch

@@ -69,7 +69,7 @@ index dca497072..454c1e7d0 100644
      }
  
 diff --git a/src/main/java/net/minecraft/server/EntityLiving.java b/src/main/java/net/minecraft/server/EntityLiving.java
-index 14637be49..dec4b442c 100644
+index 14637be49..5ccd3ea6f 100644
 --- a/src/main/java/net/minecraft/server/EntityLiving.java
 +++ b/src/main/java/net/minecraft/server/EntityLiving.java
 @@ -0,0 +0,0 @@ public abstract class EntityLiving extends Entity {
@@ -140,7 +140,7 @@ index 14637be49..dec4b442c 100644
  
 -            this.aU = true;
 -            this.getCombatTracker().g();
-+            //this.aU = true;
++            this.aU = true; // Paper - Always set at start, unset later if cancelled - GH-1432
 +            //this.getCombatTracker().g();
 +
 +            org.bukkit.event.entity.EntityDeathEvent deathEvent = null;
@@ -178,6 +178,7 @@ index 14637be49..dec4b442c 100644
 +                this.setDying(true);
 +                this.world.broadcastEntityEffect(this, (byte) 3);
 +            } else {
++                this.setDying(false); // Paper - reset if cancelled
 +                this.setHealth((float) deathEvent.getReviveHealth());
 +            }
 +            // Paper end