Account for optional out parameters in IStream

[miloush]

Fixes #6344

Description

System.Windows.Media.StreamAsIStream.Write(byte[] buffer, uint cb, out uint cbWritten) throws NullReferenceException when trying to set cbWritten because the out int cbWritten parameter has been passed in as null.

According to the documentation, caller can pass NULL as pcbWritten to IStream::Write when they are not interested in the number of bytes written.

The CManagedStreamWrapper does not expect that and simply passes the pointer to the managed assembly, however, CLR is not happy with writing to null out parameters.

The PR fixes the issue by always passing a valid reference to the managed assembly. If an output argument was omitted, a reference to a local variable on stack is passed instead, and the output value is thrown away.

The same convention applies to IStream::Seek and IStream::CopyTo methods.

Customer Impact

Customers are unable to use WIC encoders (and potentially decoders) that call IStream without the optional parameters (i.e. passing NULL). In the case of #6344, customers cannot save HEIF files using a paid encoder provided by Microsoft (built-in in previous Windows builds).

Regression

No.

Testing

Compiled an updated wpfgfx_cor3.dll and tested in .NET 6.0.2 x86 WPF application with the repro code in #6344.

Risk

I am not aware of any risks, the fix does not affect any currently working scenario.

[lindexi]

Thank you and look good to me.