pcm_multi accepts sparse/unbound bindings and aborts via assertion

[GitHub issues - opened <github@alsa-project.org>]

alsa-project/alsa-lib issue #523 was opened from HarshRajSinghania:

### Summary

The `multi` PCM configuration parser can leave intermediate binding slots unbound (`slave_idx == -1`), but `snd_pcm_multi_open()` evaluates `schannels[i] < schannels_count[sidxs[i]]` before checking whether `sidxs[i]` is negative. A sparse binding therefore indexes `schannels_count[-1]` and, in builds with assertions enabled, aborts the process.

### Reproducer

```
pcm.null { type null }
pcm.poc {
    type multi
    slaves.a.pcm "null"
    slaves.a.channels 2
    bindings.0.slave a
    bindings.0.channel 0
    bindings.5000.slave a
    bindings.5000.channel 1
}
```

Opening `poc` with alsa-lib 1.2.14 reaches `snd_pcm_multi_open()` and terminates with:

```
pcm_multi.c:1125: snd_pcm_multi_open: Assertion `schannels[i] < schannels_count[sidxs[i]]' failed.
Aborted
```

The configuration documentation explicitly permits binding entries indexed by `N`, so sparse binding indices are representable configuration input.

### Suggested fix

Treat an unbound slot as unbound before indexing `schannels_count`, and return `-EINVAL` for an invalid slave index/channel instead of relying on assertions.

I have a patch prepared that makes these checks explicit and avoids the negative-index access/assertion abort.

Reported by Harsh Raj Singhania.

Issue URL     : https://github.com/alsa-project/alsa-lib/issues/523
Repository URL: https://github.com/alsa-project/alsa-lib