AP_GPS: Fix bug in calculation of blended GPS delay

libraries/AP_GPS/AP_GPS.cpp

@@ -1454,7 +1454,7 @@ void AP_GPS::calc_blended_state(void)
         if (_blend_weights[i] > 0.0f) {
             temp_time_1 += (double)timing[i].last_fix_time_ms * (double) _blend_weights[i];
             temp_time_2 += (double)timing[i].last_message_time_ms * (double)_blend_weights[i];
-            _blended_lag_sec += get_lag(i) * _blended_lag_sec;
+            _blended_lag_sec += get_lag(i) * _blend_weights[i];
         }
     }
     timing[GPS_BLENDED_INSTANCE].last_fix_time_ms = (uint32_t)temp_time_1;