- attempt to fix "peer disappeared" coredumps:

- attempt to fix "peer disappeared" coredumps:
  In every callback function check if callback data is valid and
  completely abort any work with that digest if it is not
  the fix may eliminate coredumps, but may leak (at least) digest memory;
  peer structure gets de-allocated in another module, and perhaps
  we should deallocate digests there as well.