Fix several off-by-one issues in UploadFileMessage

We now fuzz UploadFileMessage, generating random files and checking they
round-trip correctly.

The joy of having a long-lasting refactor branch with an absolutely
massive diff, is that you end up spotting bugs, and then it's a massive
pain to merge the fix back into trunk!

src/test/java/dan200/computercraft/support/FakeContainer.java

@@ -0,0 +1,25 @@
+/*
+ * 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.support;
+
+import net.minecraft.entity.player.PlayerEntity;
+import net.minecraft.inventory.container.Container;
+
+import javax.annotation.Nonnull;
+
+public class FakeContainer extends Container
+{
+    public FakeContainer()
+    {
+        super( null, 0 );
+    }
+
+    @Override
+    public boolean stillValid( @Nonnull PlayerEntity player )
+    {
+        return true;
+    }
+}