multiqueue: Fix hanging if shut down while handling a serialized query
multiqueue: Fix hanging if shut down while handling a serialized query

https://bugzilla.gnome.org/show_bug.cgi?id=721253

cablelabs/gstreamer,Lachann/gstreamer,Lachann/gstreamer,cablelabs/gstreamer,ensonic/gstreamer,ensonic/gstreamer,krichter722/gstreamer,jpakkane/gstreamer,cfoch/gstreamer,krichter722/gstreamer,jpakkane/gstreamer,jpakkane/gstreamer,mparis/gstreamer,surround-io/gstreamer,StreamUtils/gstreamer,ensonic/gstreamer,surround-io/gstreamer,centricular/gstreamer,cablelabs/gstreamer,StreamUtils/gstreamer,shelsonjava/gstreamer,centricular/gstreamer,centricular/gstreamer,jpakkane/gstreamer,jpxiong/gstreamer,mparis/gstreamer,shelsonjava/gstreamer,centricular/gstreamer,Lachann/gstreamer,Lachann/gstreamer,ensonic/gstreamer,mparis/gstreamer,surround-io/gstreamer,shelsonjava/gstreamer,jpxiong/gstreamer,cfoch/gstreamer,krichter722/gstreamer,jpxiong/gstreamer,cfoch/gstreamer,krichter722/gstreamer,surround-io/gstreamer,centricular/gstreamer,cfoch/gstreamer,cablelabs/gstreamer,surround-io/gstreamer,cfoch/gstreamer,cablelabs/gstreamer,shelsonjava/gstreamer,mparis/gstreamer,StreamUtils/gstreamer,mparis/gstreamer,Lachann/gstreamer,jpxiong/gstreamer,StreamUtils/gstreamer,krichter722/gstreamer,jpakkane/gstreamer,ensonic/gstreamer,jpxiong/gstreamer,StreamUtils/gstreamer,shelsonjava/gstreamer