dcadec: skip QMF on unused channels

When the extra rear channel is present but unused, the
s->channel_order_tab[] value for that channel is -1. The QMF can be
skipped for the extra channel, and doing so avoids an out-of-array read
on s->samples_chanptr[].
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
Signed-off-by: Justin Ruggles <justin.ruggles@gmail.com>

libavcodec/dcadec.c

@@ -1279,9 +1279,10 @@ static int dca_filter_channels(DCAContext *s, int block_index)
     for (k = 0; k < s->prim_channels; k++) {
 /*        static float pcm_to_double[8] = { 32768.0, 32768.0, 524288.0, 524288.0,
                                             0, 8388608.0, 8388608.0 };*/
-        qmf_32_subbands(s, k, subband_samples[k],
-                        s->samples_chanptr[s->channel_order_tab[k]],
-                        M_SQRT1_2 / 32768.0 /* pcm_to_double[s->source_pcm_res] */);
+        if (s->channel_order_tab[k] >= 0)
+            qmf_32_subbands(s, k, subband_samples[k],
+                            s->samples_chanptr[s->channel_order_tab[k]],
+                            M_SQRT1_2 / 32768.0 /* pcm_to_double[s->source_pcm_res] */);
     }
 
     /* Down mixing */