Do not fire EntityDamageEvent for explosions twice. Fixes BUKKIT-5156

Explosions directly caused by LivingEntities, such as creepers and tnt lit
by players, have their EntityDamageEvent explicitely handled within
the Explosion class.  In order to prevent double events when damage
is handled for other DamageSources, we need return null for explosion
based damage sources.

src/main/java/org/bukkit/craftbukkit/event/CraftEventFactory.java

@@ -389,7 +389,10 @@ public class CraftEventFactory {
     }
 
     public static EntityDamageEvent handleEntityDamageEvent(Entity entity, DamageSource source, float damage) {
-        if (source instanceof EntityDamageSource) {
+        // Should be isExplosion
+        if (source.c()) {
+            return null;
+        } else if (source instanceof EntityDamageSource) {
             Entity damager = source.getEntity();
             DamageCause cause = DamageCause.ENTITY_ATTACK;