Give FakeNetworkManager a nonnull channel.

Another mod was using mixins to access this private field without any
null checking. I got the idea to use an EmbeddedChannel from Carpet,
which was having the same issue with this mod.

Fixes #53.

src/main/java/dan200/computercraft/fabric/mixin/ConnectionAccess.java

@@ -0,0 +1,18 @@
+/*
+ * This file is part of ComputerCraft - http://www.computercraft.info
+ * Copyright Daniel Ratcliffe, 2011-2022. Do not distribute without permission.
+ * Send enquiries to dratcliffe@gmail.com
+ */
+package dan200.computercraft.fabric.mixin;
+
+import io.netty.channel.Channel;
+import net.minecraft.network.Connection;
+import org.spongepowered.asm.mixin.Mixin;
+import org.spongepowered.asm.mixin.gen.Accessor;
+
+@Mixin( Connection.class )
+public interface ConnectionAccess
+{
+    @Accessor
+    void setChannel( Channel channel );
+}

src/main/java/dan200/computercraft/shared/util/FakeNetHandler.java

@@ -6,7 +6,9 @@
 package dan200.computercraft.shared.util;
 
 import dan200.computercraft.api.turtle.FakePlayer;
+import dan200.computercraft.fabric.mixin.ConnectionAccess;
 import io.netty.channel.ChannelHandlerContext;
+import io.netty.channel.embedded.EmbeddedChannel;
 import io.netty.util.concurrent.Future;
 import io.netty.util.concurrent.GenericFutureListener;
 import net.minecraft.network.Connection;
@@ -283,6 +285,7 @@ public class FakeNetHandler extends ServerGamePacketListenerImpl
         FakeNetworkManager()
         {
             super( PacketFlow.CLIENTBOUND );
+            ((ConnectionAccess)this).setChannel( new EmbeddedChannel() );
         }
 
         @Override

src/main/resources/computercraft.mixins.json

@@ -3,6 +3,7 @@
     "package": "dan200.computercraft.fabric.mixin",
     "compatibilityLevel": "JAVA_16",
     "mixins": [
+        "ConnectionAccess",
         "LevelResourceAccess",
         "MinecraftServerAccess",