don't go below 0 for pickupDelay, breaks picking up items

vanilla checks for == 0

Fixes #1911

Spigot-Server-Patches/PlayerPickupItemEvent-setFlyAtPlayer.patch

@@ -5,7 +5,7 @@ Subject: [PATCH] PlayerPickupItemEvent#setFlyAtPlayer
 
 
 diff --git a/src/main/java/net/minecraft/server/EntityItem.java b/src/main/java/net/minecraft/server/EntityItem.java
-index 39a804b7c..643793d55 100644
+index 39a804b7c..921a56c31 100644
 --- a/src/main/java/net/minecraft/server/EntityItem.java
 +++ b/src/main/java/net/minecraft/server/EntityItem.java
 @@ -0,0 +0,0 @@ public class EntityItem extends Entity {
@@ -31,9 +31,10 @@ index 39a804b7c..643793d55 100644
                  }
  
 @@ -0,0 +0,0 @@ public class EntityItem extends Entity {
+             // CraftBukkit end
  
              if (this.pickupDelay == 0 && (this.g == null || 6000 - this.age <= 200 || this.g.equals(entityhuman.getUniqueID())) && entityhuman.inventory.pickup(itemstack)) {
-                 entityhuman.receive(this, i);
+-                entityhuman.receive(this, i);
 +                // Paper Start
 +                if (flyAtPlayer) {
 +                    entityhuman.receive(this, i);