avcodec/dcadec: Check that the added xch channel isnt already there

Fixes null pointer dereference
Fixes: signal_sigsegv_369609d_623_cov_2008234281_ES_6.1_16bit.dts
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 7d593495e4)

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/dcadec.c

@@ -2359,6 +2359,10 @@ FF_ENABLE_DEPRECATION_WARNINGS
 #else
             if (s->xch_present && !s->xch_disable) {
 #endif
+                if (avctx->channel_layout & AV_CH_BACK_CENTER) {
+                    avpriv_request_sample(avctx, "XCh with Back center channel");
+                    return AVERROR_INVALIDDATA;
+                }
                 avctx->channel_layout |= AV_CH_BACK_CENTER;
                 if (s->lfe) {
                     avctx->channel_layout |= AV_CH_LOW_FREQUENCY;