mpegaudiodec: copy AVFloatDSPContext from first context to all contexts

This fixes a segfault when decoding multi-channel MP3onMP4 files.

This is similar to commit cb72230d for MPADSPContext.
Reviewed-by: Michael Niedermayer <michaelni@gmx.at>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>

libavcodec/mpegaudiodec_template.c

@@ -1893,6 +1893,7 @@ static av_cold int decode_init_mp3on4(AVCodecContext * avctx)
         s->mp3decctx[i]->adu_mode = 1;
         s->mp3decctx[i]->avctx = avctx;
         s->mp3decctx[i]->mpadsp = s->mp3decctx[0]->mpadsp;
+        s->mp3decctx[i]->fdsp = s->mp3decctx[0]->fdsp;
     }
 
     return 0;