nl_recv(): Memory allocation errors are handled properly now

nl_recv(): Memory allocation errors are handled properly now

1. all cleanup actions (like free()) now located at the end of function
2. in case of error or EOF, *buf and *creds (if given) set to NULL
   This protect from invalid code at user's side, like:
   char *buf;
   x = nl_recv(..., &buf, ...);
   if (x<=0)
      goto cleanup;
   cleanup:
      free(buf);
3. all intermediate buffers are stored into local variables, and user's
   variables only touches at the end.