pcm: reject invalid multi bindings without assertions

[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.