AC_Precland: bug fix to update ef angles only with new readings

libraries/AC_PrecLand/AC_PrecLand.cpp

@@ -132,11 +132,13 @@ void AC_PrecLand::calc_angles_and_pos(float alt_above_terrain_cm)
     // exit immediately if not enabled
     if (_backend == NULL) {
         _have_estimate = false;
+        return;
     }
 
     // get body-frame angles to target from backend
     if (!_backend->get_angle_to_target(_bf_angle_to_target.x, _bf_angle_to_target.y)) {
         _have_estimate = false;
+        return;
     }
 
     // subtract vehicle lean angles