Fixes crash based on a uninitialized array index read.
If the read does not crash then out of array writes based
on the same index might have been triggered afterwards.

Found-by: inferno@chromium.org
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 9e1c55cf

)
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>