pcm: reject invalid multi bindings without assertions

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

alsa-project/alsa-lib pull request #524 was opened from HarshRajSinghania:

Fixes #523

The `multi` PCM parser initializes unused binding slots with `slave_idx == -1`. `snd_pcm_multi_open()` previously evaluated `schannels[i] < schannels_count[sidxs[i]]` before checking for an unbound slot, causing a negative array index and an assertion abort for sparse binding configurations.

This patch:

* skips unbound binding slots before dereferencing the slave index;
* validates the slave index and channel explicitly;
* returns `-EINVAL` instead of relying on assertions for malformed configuration.

Reproducer from #523:

```
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
}
```

Before the patch, alsa-lib aborts with:

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

The fix makes the malformed configuration fail cleanly with `-EINVAL`.

Reported and fixed by Harsh Raj Singhania.

Request URL   : https://github.com/alsa-project/alsa-lib/pull/524
Patch URL     : https://github.com/alsa-project/alsa-lib/pull/524.patch
Repository URL: https://github.com/alsa-project/alsa-lib