Fix useQuixMix attribute
Fix useQuixMix attribute

No station was shown as useQuickMix, because callback function set
pointer to NULL for members that did not have anything to do with the
quickmix stuff. Don't do that.

Closes #118.

tchajed/pianobar,DavidZemon/pianobar,bmbove/pianobar-save,tchajed/pianobar,tchajed/pianobar,tchajed/pianobar,Vi1i/pianobarfly,johnso51/pianobar,Vi1i/pianobarfly,johndeeney/pianobar,twahlfeld/pianobar,twahlfeld/pianobar,johndeeney/pianobar,gnowxilef/pianobar,twahlfeld/pianobar,twahlfeld/pianobar,bmbove/pianobar-save,bmbove/pianobar-save,johndeeney/pianobar,tchajed/pianobar,Vi1i/pianobarfly,johndeeney/pianobar,bmbove/pianobar-save,gnowxilef/pianobar,bmbove/pianobar-save,Vi1i/pianobarfly,johndeeney/pianobar,tchajed/pianobar,twahlfeld/pianobar,johnso51/pianobar,johndeeney/pianobar,gnowxilef/pianobar,gnowxilef/pianobar,DavidZemon/pianobar,gnowxilef/pianobar,Vi1i/pianobarfly,johnso51/pianobar,DavidZemon/pianobar,twahlfeld/pianobar,johnso51/pianobar,DavidZemon/pianobar,gnowxilef/pianobar,DavidZemon/pianobar,DavidZemon/pianobar,johnso51/pianobar,johnso51/pianobar