secure pollcallback against self-removal

when a pollcallback removes itself from the pollqueue, we got crashes
TODO: we could still get crashes if the 'next' entry get's removed. in theory this
should be impossible, as objects should only remove their own callbacks, so the removal
of 'next' could not be scheduled until it is actually called (at which point it is 'current'
and not 'next' anymore)