dsound: don't free buffer if it's owned by alsa

[Dan Kegel]

This fixes the repeatable double free reported in
http://bugs.winehq.org/show_bug.cgi?id=13723
Found via Valgrind.  Thanks to Maarten for suggesting
how to decide whether we need to free the buffer.
Given how close we are to release, maybe somebody
should doublecheck this.
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