[5/7] windowscodecs: Add initial implementation of IWICMetadataQueryReader::GetMetadataByName.

[Vincent Povirk]

> +        if (vt == VT_CLSID)
> +        {
> +            id->u.puuid = CoTaskMemAlloc(sizeof(GUID));
> +            if (!id->u.puuid) return E_OUTOFMEMORY;
> +
> +            hr = UuidFromStringW(next_token.u.bstrVal, id->u.puuid);
> +        }
> +        else
> +            hr = PropVariantChangeType(id, &next_token, 0, vt);
> +        PropVariantClear(&next_token);
> +        if (hr != S_OK)
> +        {
> +            PropVariantClear(id);
> +            PropVariantClear(schema);
> +            return hr;
> +        }
> +
> +        id->vt = vt;

Assigning the vt at the end of this is a bit strange. I don't know any
situation here where PropVariantChangeType will succeed without giving
you the requested vt, and if it did, changing it would be unsafe.

I can see you need it in the VT_CLSID case, but then it would make
more sense to set it there. In fact, this version will leak memory if
UuidFromStringW fails.

+        if (!elem.len) break;

When can this happen?

+            new_value.vt = VT_UNKNOWN;
+            hr = MetadataQueryReader_CreateInstance(This->block,
root, (IWICMetadataQueryReader **)&new_value.u.punkVal);

I think you need to clear new_value first here?