Fix MemoryStreamArchiveReader.GetStream() failing in some cases
`MemoryStreamArchiveReader` introduced in
0657b55196 would previously use
`MemoryStream.GetBuffer()` to retrieve the underlying byte buffer with
stream data. However, this is not generally the method you would want,
for two reasons:
1. It can fail if the stream wasn't created in the way that supports it.
2. As per
https://learn.microsoft.com/en-us/dotnet/api/system.io.memorystream.getbuffer?view=net-7.0#system-io-memorystream-getbuffer,
it will return the _raw_ contents of the buffer, including
potentially unused bytes.
To fix, use `MemoryStream.ToArray()` instead, which avoids both
pitfalls. Notably, `ToArray()` always returns the full contents of the
buffer, regardless of `Position`, as documented in:
https://learn.microsoft.com/en-us/dotnet/api/system.io.memorystream.toarray?view=net-7.0#system-io-memorystream-toarray
e57d7d1205