dca: Respect the current limits in the downmixing capabilities

Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org

libavcodec/dcadec.c

@@ -807,6 +807,12 @@ static int dca_subframe_header(DCAContext *s, int base_channel, int block_index)
                        "Invalid channel mode %d\n", am);
                 return AVERROR_INVALIDDATA;
             }
+            if (s->prim_channels > FF_ARRAY_ELEMS(dca_default_coeffs[0])) {
+                avpriv_request_sample(s->avctx, "Downmixing %d channels",
+                                      s->prim_channels);
+                return AVERROR_PATCHWELCOME;
+            }
+
             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];