Add an extra check in test_entrynodes

This check makes it so we can reach "done" without setting "conn",
and so the "if (conn)" check will not be redundant, and so coverity
won't complain.  Fixes CID 1422205.  Not actually a bug.

src/test/test_entrynodes.c

@@ -2721,6 +2721,8 @@ test_entry_guard_outdated_dirserver_exclusion(void *arg)
     smartlist_add(digests, (char*)prose);
   }
 
+  tt_int_op(smartlist_len(digests), OP_EQ, 20);
+
   /* ... now mock some functions */
   mock_ns_val = tor_malloc_zero(sizeof(networkstatus_t));
   MOCK(networkstatus_get_latest_consensus_by_flavor, mock_ns_get_by_flavor);