Clear containers on cancelled place events

It is possible for a container to be brought into the world pre-filled
in place event (thanks NBT!). When the blockplacevent(s) are cancelled
their blockstate is updated in OBC. This triggers them to drop their
inventories.

Rather than screwing with the update method which is used elsewhere, we
can simply clear any inventories once we are told that a plugin intends
to cancel the interaction.

Fixes GH-1831

Spigot-Server-Patches/0436-Clear-containers-when-block-place-is-cancelled.patch

@@ -0,0 +1,26 @@
+From 86d1c4ec47db13ba21b98d644aee08f792fc20dd Mon Sep 17 00:00:00 2001
+From: Zach Brown <zach@zachbr.io>
+Date: Sat, 6 Apr 2019 00:30:32 -0400
+Subject: [PATCH] Clear containers when block place is cancelled
+
+Fixes GH-1831
+
+diff --git a/src/main/java/net/minecraft/server/ItemStack.java b/src/main/java/net/minecraft/server/ItemStack.java
+index d8ebcf0a3..0ef928a2b 100644
+--- a/src/main/java/net/minecraft/server/ItemStack.java
++++ b/src/main/java/net/minecraft/server/ItemStack.java
+@@ -246,6 +246,11 @@ public final class ItemStack {
+                     for (Map.Entry<BlockPosition, TileEntity> e : world.capturedTileEntities.entrySet()) {
+                         if (e.getValue() instanceof TileEntityLootable) {
+                             ((TileEntityLootable) e.getValue()).setLootTable(null);
++
++                            // Clear containers before updating their blockstate below - Fixes GH-1831
++                            if (e.getValue() instanceof TileEntityContainer) {
++                                ((TileEntityContainer) e.getValue()).clear();
++                            }
+                         }
+                     }
+                     // Paper end
+-- 
+2.21.0
+