ac3dec: update checked AV_EF flags.

A user who wishes to use default error concealment cannot set the
AV_EF_CRCCHECK flag because not every CRC in every format is a
reliable indicator of bitstream damage. In some formats crcrs
can be nonsensical in absence of any damage. We thus add the
AV_EF_CAREFUL flag in addition to the AV_EF_CRCCHECK flag to
allow a user to enable this reliable CRC check without having to
enable all CRC checks in all formats.

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