Fix vad leaks

[silviucpp]

This patch fixes multiple issues:

• memory leak because vad_segments is never released
• avoid segmentation fault when whisper_vad_segments_from_samples returns nullptr.
• avoid potential segmentation fault when the app fails to allocate memory for filtered samples and the vad context is released but also get released withing state itself when whisper_free_state is called

[danbev]

@silviucpp Thank you!