package/libvorbis: fix CVE-2018-10392

mapping0_forward in mapping0.c in Xiph.Org libvorbis 1.3.6 does not
validate the number of channels, which allows remote attackers to cause
a denial of service (heap-based buffer overflow or over-read) or
possibly have unspecified other impact via a crafted file.

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>