ffmdec: initialize f_cprv, f_stvi and f_stau

They are used in a switch statement, but it is not guaranteed that the
COMM case (where they are set to 0) is reached before the other cases.

Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
