Additionally remove Mob target if the target is removed

If the target is removed, then it is possible that
using the CraftEntity could retrieve a teleported
entity. This would cause the Mob to be removed
when getHandle() is invoked.

Fixes #117

patches/server/0003-Threaded-Regions.patch

@@ -17872,7 +17872,7 @@ index e11d7283662834047b2ff81a2fd25a4263792deb..40dd667bacd296a3a329391dc87a5713
 
              while (!flag2 && blockposition.getY() > world.getMinBuildHeight()) {
 diff --git a/src/main/java/net/minecraft/world/entity/Mob.java b/src/main/java/net/minecraft/world/entity/Mob.java
-index e2a25c29ec74147b3e66aa0b3deb85a8f6ee53a5..a8b23b1594d2b39568c68c93a8a1b936457672bc 100644
+index e2a25c29ec74147b3e66aa0b3deb85a8f6ee53a5..a21edc7df32a59a6cb1af3865d2ce35f72cf8afc 100644
 --- a/src/main/java/net/minecraft/world/entity/Mob.java
 +++ b/src/main/java/net/minecraft/world/entity/Mob.java
 @@ -134,6 +134,14 @@ public abstract class Mob extends LivingEntity implements Targeting {
@@ -17895,7 +17895,7 @@ index e2a25c29ec74147b3e66aa0b3deb85a8f6ee53a5..a8b23b1594d2b39568c68c93a8a1b936
      @Override
      public LivingEntity getTarget() {
 +        // Folia start - region threading
-+        if (!io.papermc.paper.util.TickThread.isTickThreadFor(this.target)) {
++        if (this.target != null && (!io.papermc.paper.util.TickThread.isTickThreadFor(this.target) || this.target.isRemoved())) {
 +            this.target = null;
 +            return null;
 +        }