DataValidator: fix unexpected failover

Instead of always starting with instance 0 (potentially an internal
mag), first take the current sensor as reference to compare the other
ones against it.
The issue is that otherwise we can end up in a
situation where a switch occurs because the currently used sensor isn't
much better than a sensor with a lower index: because the selected one
isn't much better, we cannot "fail-over" to it and get stuck on another
sensor, triggering a fail-over.

src/modules/sensors/data_validator/DataValidatorGroup.cpp

@@ -153,14 +153,32 @@ float *DataValidatorGroup::get_best(uint64_t timestamp, int *index)
 
 	int i = 0;
 
+	// First find the current selected sensor
+	while (next != nullptr) {
+		if (i == pre_check_best) {
+			const int prio = next->priority();
+			const float confidence = next->confidence(timestamp);
+
+			pre_check_prio = prio;
+			pre_check_confidence = confidence;
+
+			max_index = i;
+			max_confidence = confidence;
+			max_priority = prio;
+			best = next;
+			break;
+		}
+
+		next = next->sibling();
+		i++;
+	}
+
+	i = 0;
+	next = _first;
+
 	while (next != nullptr) {
 		float confidence = next->confidence(timestamp);
 
-		if (i == pre_check_best) {
-			pre_check_prio = next->priority();
-			pre_check_confidence = confidence;
-		}
-
 		/*
 		 * Switch if:
 		 * 1) the confidence is higher and priority is equal or higher