dsound: Enforce invariant about BlockAlign and nAvgBytesPerSec.

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

Hi,

this second dsound patch depends on yesterday's modification
 (Get rid of redundant NULL check).
It is enough to fix bug 12349 and bug 19901, yet
it's not the final word on winmm blockalign issues.

Regards,
	Jörg Höhle
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