Guard against non-living entities in MACreature.

This should prevent runtime errors where MobArena tries to spawn non-living entities, causing arenas to get locked up.

Fixes #438

src/main/java/com/garbagemule/MobArena/waves/MACreature.java

@@ -29,10 +29,12 @@ public class MACreature
     static {
         map = new HashMap<>();
         for (EntityType type : EntityType.values()) {
+            if (type.isAlive()) {
                 // Instantiating a new creature registers it
                 new MACreature(type);
             }
         }
+    }
 
     // Default creatures
     public static final MACreature ZOMBIE = new MACreature("zombie", EntityType.ZOMBIE);