mp3on4: copy MPADSPContext from first context to all contexts.

Fixes segfault when decoding multi-channel MP3onMP4 files.

libavcodec/mpegaudiodec.c

@@ -1959,6 +1959,7 @@ static int decode_init_mp3on4(AVCodecContext * avctx)
         s->mp3decctx[i] = av_mallocz(sizeof(MPADecodeContext));
         s->mp3decctx[i]->adu_mode = 1;
         s->mp3decctx[i]->avctx = avctx;
+        s->mp3decctx[i]->mpadsp = s->mp3decctx[0]->mpadsp;
     }
 
     return 0;