Use Aurora writer for reads if max_lag_ms isn't specified

This solves the issue described in issue #3522

It is a side effect of a mistake in commit 5336e72f1c

lib/MySQL_HostGroups_Manager.cpp

@@ -2843,7 +2843,7 @@ MySrvC *MyHGC::get_random_MySrvC(char * gtid_uuid, uint64_t gtid_trxid, int max_
 				}
 			}
 		}
-		if (max_lag_ms) { // we are using AWS Aurora, as this logic is implemented only here
+		if (max_lag_ms > 0) { // we are using AWS Aurora, as this logic is implemented only here
 			unsigned int min_num_replicas = sess->thread->variables.aurora_max_lag_ms_only_read_from_replicas;
 			if (min_num_replicas) {
 				if (num_candidates >= min_num_replicas) { // there are at least N replicas