sndio: fix crash when volume is adjusted while playback is stopped

An use-after-free caused by calls to the mute/volume functions after the handle is freed.

http://cvsweb.openbsd.org/cgi-bin/cvsweb/~~checkout~~/ports/x11/vlc/patches/patch-modules_audio_output_sndio_c?rev=1.1&content-type=text/plain&hideattic=1

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