avcodec/ac3dec: Check expacc

this is somewhat a magic number, which can be understood from reading section
"7.1.2 Exponent Strategy" of the ac3 specification, in short:
Three exponents each represented as number 0-4 are grouped together and
base-5 encoded, so the maximal correct value is 25*4 + 5*4 + 4 = 124.

Reviewed-by: Andreas Cadhalpun <andreas.cadhalpun@googlemail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
