vorbisdec: Error on bark_map_size equal to 0.

      The value is used to calculate output LSP curve and a division by zero
and out of array accesses would occur.

CVE-2013-0894

CC: libav-stable@libav.org

Reported-by: Dale Curtis <dalecurtis@chromium.org>
Found-by: inferno@chromium.org
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>