Fix an item leak, that happens when some node calls ng_callout() two
times consequently, without checking whether callout has been serviced
or not. (ng_pptpgre and ng_ppp were catched in this behavior).

  - In ng_callout() save old item before calling callout_reset(). If the
    latter has returned 1, then free this item.
  - In ng_uncallout() clear c->c_arg.

Problem reported by:    Alexandre Kardanev