dcadec: fix global array overread.

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/dca.c

@@ -805,7 +805,7 @@ static int dca_subframe_header(DCAContext *s, int base_channel, int block_index)
                        "Invalid channel mode %d\n", am);
                 return AVERROR_INVALIDDATA;
             }
-            for (j = base_channel; j < s->prim_channels; j++) {
+            for (j = base_channel; j < FFMIN(s->prim_channels, FF_ARRAY_ELEMS(dca_default_coeffs[am])); j++) {
                 s->downmix_coef[j][0] = dca_default_coeffs[am][j][0];
                 s->downmix_coef[j][1] = dca_default_coeffs[am][j][1];
             }