dsound: Make sure we're holding the lock on Drop And Stop

[Maarten Lankhorst]

This fixes an interesting deadlock with the alsa driver:

DSOUND_Stop is called, message is added to thread queue.
DSOUND_Release is called, message is added to thread queue.

Both are fast messages, so second is added before first.

thread reads: We're going down! And terminates.

DSOUND_Stop's message is never read and it waits till eternity, so
starting/stopping never occurs any more.

Triggered this bug with my dsoundtest program.
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