tasks/lalinference.py: Fix psd-length determination logic for high-latency frame data

Since each high-frequency frame data file contains 4096s data, available_segment can be much longer than expected and can cause problems, especially invalidate an assumption that the last segment in the segment list https://git.ligo.org/soichiro.morisaki/gwcelery/blob/b93431fe061e94ff6d4a4bd52ff111b35a4488fd/gwcelery/tasks/lalinference.py#L174 contains signal.