winmm: don't convert return string in mciSendStringA on error

[Dan Kegel]

Fixes http://bugs.winehq.org/show_bug.cgi?id=17518 the
way Alexandre wanted it fixed
( http://www.winehq.org/pipermail/wine-devel/2009-March/073702.html ).

This gets rid of one todo_wine and one crash on WINEDEBUG=warn+heap make test.
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