geofence: disable altitude check if not configured

It currently cannot be configured via mavlink mission protocol.

src/modules/navigator/geofence.cpp

@@ -260,9 +260,11 @@ bool Geofence::checkPolygons(double lat, double lon, float altitude)
 	}
 
 	/* Vertical check */
+	if (_altitude_max > _altitude_min) { // only enable vertical check if configured properly
 		if (altitude > _altitude_max || altitude < _altitude_min) {
 			return false;
 		}
+	}
 
 	/* Horizontal check: iterate all polygons */
 	bool outside_exclusion = true;