The problem is, that it does not set some variables in the base class at a point.
The correct code would be:

void InFilterStream::Out(const void *p, int size)
{
	const byte *s = (byte *)p;
	if(todo) {
		dword sz = min(todo, (dword)size);
		memcpy8(t, s, sz);
		t += sz;
		s += sz;
		todo -= sz;
		size -= sz;
		pos += sz;
	}
	if(size) {
		int l = buffer.GetCount();
		buffer.SetCountR(l + size);
		memcpy8(buffer.begin() + l, s, size);
		Stream::buffer = ptr = buffer.begin();
		rdlim = buffer.end();
	}
	WhenOut();
}

The added two lines are:

+		Stream::buffer = ptr = buffer.begin();
+		rdlim = buffer.end();

I hope, it will not break any other things in code.

I found it, when I wanted to use that as a gzip input filter to read from an uncompressed stream in gzipped format:

class MyGzipInStream : public InFilterStream {
	Zlib         z;

public:
	void Open(Stream& in)    { Set(in, z); z.Compress(); }
	Zlib& GetZlib()           { return z; }
	MyGzipInStream()    { z.GZip().Header(); }
	~MyGzipInStream()   { Close(); }
};