Added workaround for #WORLDGUARD-2299, java.lang.IllegalArgumentException: Instant potions cannot be extended.

src/main/java/com/sk89q/worldguard/bukkit/BukkitUtil.java

@@ -152,6 +152,17 @@ public static boolean isWaterPotion(ItemStack item) {
         return (item.getDurability() & 0x3F) == 0;
     }
 
+    /**
+     * Get just the potion effect bits. This is to work around bugs with potion
+     * parsing.
+     *
+     * @param item item
+     * @return new bits
+     */
+    public static int getPotionEffectBits(ItemStack item) {
+        return item.getDurability() & 0x3F;
+    }
+
     /**
      * Find a position for the player to stand that is not inside a block.
      * Blocks above the player will be iteratively tested until there is

src/main/java/com/sk89q/worldguard/bukkit/WorldGuardBlockListener.java

@@ -824,7 +824,7 @@ public void onBlockDispense(BlockDispenseEvent event) {
         if (wcfg.blockPotions.size() > 0) {
             ItemStack item = event.getItem();
             if (item.getType() == Material.POTION && !BukkitUtil.isWaterPotion(item)) {
-                Potion potion = Potion.fromItemStack(item);
+                Potion potion = Potion.fromDamage(BukkitUtil.getPotionEffectBits(item));
                 for (PotionEffect effect : potion.getEffects()) {
                     if (potion.isSplash() && wcfg.blockPotions.contains(effect.getType())) {
                         event.setCancelled(true);

src/main/java/com/sk89q/worldguard/bukkit/WorldGuardPlayerListener.java

@@ -412,7 +412,7 @@ public void onPlayerInteract(PlayerInteractEvent event) {
             if (item != null && item.getType() == Material.POTION && !BukkitUtil.isWaterPotion(item)) {
                 PotionEffect blockedEffect = null;
 
-                Potion potion = Potion.fromItemStack(item);
+                Potion potion = Potion.fromDamage(BukkitUtil.getPotionEffectBits(item));
                 for (PotionEffect effect : potion.getEffects()) {
                     if (wcfg.blockPotions.contains(effect.getType())) {
                         blockedEffect = effect;