Fix a memleak in nodelist_assert_ok()

  o Minor bugfixes:

    - Prevent using negative indices during unit test runs when read_all()

      fails. Spotted by coverity. Bugfix on 0.2.3.1.

    - Fix a rare memory leak when checking the nodelist without it being

      present. Found by coverity. Bugfix on 0.2.3.1.

  o Code simplifications and refactoring:

    - Remove a dead variable in the heartbeat log code. Spotted by coverity.

{

  routerlist_t *rl = router_get_routerlist();

  networkstatus_t *ns = networkstatus_get_latest_consensus();

  digestmap_t *dm = digestmap_new();

  digestmap_t *dm;

  if (!the_nodelist)

    return;

  dm = digestmap_new();

  /* every routerinfo in rl->routers should be in the nodelist. */

  if (rl) {

    SMARTLIST_FOREACH_BEGIN(rl->routers, routerinfo_t *, ri) {