af_hrtf: Fix out-of-range read.

Based on patch by Yuriy Kaminskiy [yumkam gmail].

git-svn-id: svn://svn.mplayerhq.hu/mplayer/trunk@37330 b3059339-0415-0410-9bf9-f77b7e298cf2
Signed-off-by: wm4 <wm4@nowhere>