music: set asyncmsgq to NULL when detaching sink input.
music: set asyncmsgq to NULL when detaching sink input.

This prevents asyncmsgq to have any bogus data values
when sink input needs to be re-attached.

jusa/pulseaudio-modules-nemo,maemo-foss/maemo-multimedia-pulseaudio-modules-meego,jusa/pulseaudio-modules-nemo,nemomobile/pulseaudio-module-cmtspeech-n9xx,nemomobile/pulseaudio-module-cmtspeech-n9xx,maemo-foss/maemo-multimedia-pulseaudio-modules-meego,jusa/pulseaudio-modules-nemo,maemo-foss/maemo-multimedia-pulseaudio-modules-meego