e mixer - fix segv on desklock if default sink not set yet
e mixer - fix segv on desklock if default sink not set yet

rvandegrift/e,tasn/enlightenment,tizenorg/platform.upstream.enlightenment,rvandegrift/e,tizenorg/platform.upstream.enlightenment,FlorentRevest/Enlightenment,FlorentRevest/Enlightenment,tasn/enlightenment,rvandegrift/e,tizenorg/platform.upstream.enlightenment,tasn/enlightenment,FlorentRevest/Enlightenment