alsa-pcm: check if bound ctls are valid before using it.
When bound_ctl info cannot be read this array elem info is set to NULL in 'fetch_bind_ctl'. So when we iterate the bound_ctl array we always have to check this.
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When bound_ctl info cannot be read this array elem info is set to NULL in 'fetch_bind_ctl'. So when we iterate the bound_ctl array we always have to check this.