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>

libavcodec/ac3dec.c

@@ -426,6 +426,10 @@ static int decode_exponents(AC3DecodeContext *s,
     group_size = exp_strategy + (exp_strategy == EXP_D45);
     for (grp = 0, i = 0; grp < ngrps; grp++) {
         expacc = get_bits(gbc, 7);
+        if (expacc >= 125) {
+            av_log(s->avctx, AV_LOG_ERROR, "expacc %d is out-of-range\n", expacc);
+            return AVERROR_INVALIDDATA;
+        }
         dexp[i++] = ungroup_3_in_7_bits_tab[expacc][0];
         dexp[i++] = ungroup_3_in_7_bits_tab[expacc][1];
         dexp[i++] = ungroup_3_in_7_bits_tab[expacc][2];