If it doesn't fit into 12 bits it triggers an assertion.
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 2578a546)
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>