Add further array bounds checks to bark_noise_hybridmp.

Make it clear to local analysis that no out-of-bounds array
accesses are possible here.

Follow-up to CVE-2018-10393 and CVE-2017-14160.

Signed-off-by: Thomas Daede <daede003@umn.edu>
Signed-off-by: Monty <xiphmont@xiph.org>