MSACM: initialize some ACMSTREAMHEADER fields for the sake of native codecs

[Eric Pouech]

> Native msacm (WINEDLLOVERRIDES=msacm32=n) works correctly (no loops) 
> with native codecs in wine. This suggests that msacm is doing the 
> clearing, since it is using the same builtin winmm.
fair enough

-- 
Eric Pouech