Reverse the linked list of data_validator_group and maintain a first node

for search from index 0

Fixes issue PX4/Firmware#8644

validation/data_validator_group.cpp

@@ -45,19 +45,31 @@
 
 DataValidatorGroup::DataValidatorGroup(unsigned siblings) :
 	_first(nullptr),
+	_last(nullptr),
 	_curr_best(-1),
 	_prev_best(-1),
 	_first_failover_time(0),
 	_toggle_count(0)
 {
-	DataValidator *next = _first;
+	DataValidator *next = nullptr;
+	DataValidator *prev = nullptr;
 
 	for (unsigned i = 0; i < siblings; i++) {
-		next = new DataValidator(next);
+		next = new DataValidator();
+		if(i == 0) {
+			_first = next;
+		}
+		else {
+			prev->sibling(next);
+		}
+		prev = next;
 	}
 
-	_first = next;
-	_timeout_interval_us = _first->get_timeout();
+	_last = next;
+
+	if(_first) {
+		_timeout_interval_us = _first->get_timeout();
+	}
 }
 
 DataValidatorGroup::~DataValidatorGroup()
@@ -71,13 +83,14 @@ DataValidatorGroup::~DataValidatorGroup()
 
 DataValidator *DataValidatorGroup::add_new_validator()
 {
-	DataValidator *validator = new DataValidator(_first);
+	DataValidator *validator = new DataValidator();
 	if (!validator) {
 		return nullptr;
 	}
-	_first = validator;
-	_first->set_timeout(_timeout_interval_us);
-	return _first;
+	_last->sibling(validator);
+	_last = validator;
+	_last->set_timeout(_timeout_interval_us);
+	return _last;
 }
 
 void