Fix ArgumentNullException when disposing RarArchive with damaged archives

[Copilot]

When disposing a RarArchive after processing a damaged archive, ArrayPool<byte>.Shared.Return(window) throws ArgumentNullException because the window field may be null.

Changes

• Added null check in UnpackV1.Unpack.Dispose() before returning the window buffer to the array pool
• Aligns with existing pattern in UnpackV2017 implementation

// Before
if (!externalWindow)
{
    ArrayPool<byte>.Shared.Return(window);
    window = null;
}

// After  
if (!externalWindow && window is not null)
{
    ArrayPool<byte>.Shared.Return(window);
    window = null;
}

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Exception when disposing RarArchive</issue_title>
<issue_description>I ran into an exception when disposing RarArchive after processing a damaged archive.

System.ArgumentNullException: Value cannot be null. (Parameter 'array')
   at System.Buffers.SharedArrayPool`1.Return(T[] array, Boolean clearArray)
   at SharpCompress.Compressors.Rar.UnpackV1.Unpack.Dispose() in /_/SharpCompress/Compressors/Rar/UnpackV1/Unpack.cs:line 32
   at SharpCompress.Archives.Rar.RarArchive.Dispose() in /_/SharpCompress/Archives/Rar/RarArchive.cs:line 35

I checked in SharpCompress.Compressors.Rar.UnpackV1.Unpack and found:

public override void Dispose()
{
    if (!disposed)
    {
        base.Dispose();
        if (!externalWindow)
        {
            ArrayPool<byte>.Shared.Return(window);
            window = null;
        }
        rarVM.Dispose();
        disposed = true;
    }
}

since "window" was "null" at this point I added a check fo this:

public override void Dispose()
{
    if (!disposed)
    {
        base.Dispose();
        if (!externalWindow && window is not null)
        {
            ArrayPool<byte>.Shared.Return(window);
            window = null;
        }
        rarVM.Dispose();
        disposed = true;
    }
}
```</issue_description>

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

• Fixes Exception when disposing RarArchive #1020

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Pull Request Overview

This PR fixes an ArgumentNullException that occurs when disposing a RarArchive after processing a damaged archive. The issue arises because the window field may be null when Dispose() is called, but the code attempts to return it to the ArrayPool without checking for null first.

• Added defensive null check in UnpackV1.Unpack.Dispose() before returning the window buffer to the ArrayPool
• Aligns with existing null-check patterns used in RarVM.cs, LzOutWindow.cs, and UnpackV2017

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