oma: Fix out of array read.

Input: 01-Untitled-partial.oma
ZZUF params: zzuf[s=7157,r=0.001]

Fixes Bugzilla #106

Bug-found-by: darkshikari
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
Signed-off-by: Ronald S. Bultje <rsbultje@gmail.com>
