avcodec/ac3dec_fixed: fix compilation when ac3dec is disabled

Reviewed-by: Michael Niedermayer <michaelni@gmx.at>
Signed-off-by: James Almer <jamrial@gmail.com>

libavcodec/ac3dec.c

@@ -196,10 +196,10 @@ static av_cold int ac3_decode_init(AVCodecContext *avctx)
     s->fdsp = avpriv_alloc_fixed_dsp(avctx->flags & CODEC_FLAG_BITEXACT);
 #else
     s->fdsp = avpriv_float_dsp_alloc(avctx->flags & CODEC_FLAG_BITEXACT);
+    ff_fmt_convert_init(&s->fmt_conv, avctx);
 #endif
 
     ff_ac3dsp_init(&s->ac3dsp, avctx->flags & CODEC_FLAG_BITEXACT);
-    ff_fmt_convert_init(&s->fmt_conv, avctx);
     av_lfg_init(&s->dith_state, 0);
 
     if (USE_FIXED)