Remove an assert in StgStreamImpl_Write()

[Robert Shearman]

Francois Gouget wrote:

>
> Changelog:
>
>  * dlls/ole32/stg_stream.c
>
>    Aric Stewart <aric at codeweavers.com>
>    Remove an assert and instead just fail.
>    This allows iTunes to finish installing with the reboot.
>

A better fix would be to check the result of IStream_SetSize in this 
function, although I think the original problem may well be fixed after 
the STG_E_REVERTED checks.

-- 
Rob Shearman