wineoss: Properly allocate the WAVEFORMATEXTENSIBLE returned by GetMixFormat.

[Joerg-Cyril.Hoehle at t-systems.com]

Hi,

The only thing that prevented me from merging these patches is that none
of my partitions has a system with OSS4 :-(
Thus this patch has never been compiled.
And I'm not able to test Wine anymore with wineoss.

I'm considering moving to Debian unstable...

Regards,
	Jörg Höhle