prores: Evaluate all the quantizers

Prevent an uninitialized data access.

CC: libav-stable@libav.org
Bug-Id: CID 703824 / CID 703825
Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
