takdec: ensure chan2 is a valid channel index

If chan2 is not smaller than the number of channels, it can cause
segmentation faults due to dereferencing a NULL pointer.
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/takdec.c

@@ -801,6 +801,12 @@ static int tak_decode_frame(AVCodecContext *avctx, void *data,
                     if (s->mcdparams[i].present) {
                         s->mcdparams[i].index = get_bits(gb, 2);
                         s->mcdparams[i].chan2 = get_bits(gb, 4);
+                        if (s->mcdparams[i].chan2 >= avctx->channels) {
+                            av_log(avctx, AV_LOG_ERROR,
+                                   "invalid channel 2 (%d) for %d channel(s)\n",
+                                   s->mcdparams[i].chan2, avctx->channels);
+                            return AVERROR_INVALIDDATA;
+                        }
                         if (s->mcdparams[i].index == 1) {
                             if ((nbit == s->mcdparams[i].chan2) ||
                                 (ch_mask & 1 << s->mcdparams[i].chan2))