pcm: replace multi plugin binding asserts with -EINVAL checks

pcm: replace multi plugin binding asserts with -EINVAL checks

Sparse binding maps leave sidxs[i] == -1 for gap indices. The previous
assert() calls caused SIGABRT in debug builds and OOB reads under NDEBUG.
Return -EINVAL with explicit bounds and sign checks instead.

[ fixed coding style -- tiwai ]

Closes: https://github.com/alsa-project/alsa-lib/pull/527
Signed-off-by: Harsh Raj Singhania <raj.harshraut@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>