dca: dont overread dca_default_coeffs.

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

libavcodec/dca.c

@@ -801,10 +801,14 @@ static int dca_subframe_header(DCAContext *s, int base_channel, int block_index)
             }
         } else {
             int am = s->amode & DCA_CHANNEL_MASK;
+            if (am < 16) {
             for (j = base_channel; j < s->prim_channels; j++) {
                 s->downmix_coef[j][0] = dca_default_coeffs[am][j][0];
                 s->downmix_coef[j][1] = dca_default_coeffs[am][j][1];
             }
+            } else {
+                av_log(s->avctx, AV_LOG_WARNING, "amode > 15 default downmix_coef unsupported\n");
+            }
         }
     }