aaccoder_twoloop: Mark sfdiff as av_unused

Silences warning when building without assertions
Signed-off-by: Claudio Freire <klaussfreire@gmail.com>

libavcodec/aaccoder_twoloop.h

@@ -691,7 +691,7 @@ static void search_for_quantizers_twoloop(AVCodecContext *avctx,
             /** Check that there's no SF delta range violations */
             if (!sce->zeroes[w*16+g]) {
                 if (prev != -1) {
-                    int sfdiff = sce->sf_idx[w*16+g] - prev + SCALE_DIFF_ZERO;
+                    av_unused int sfdiff = sce->sf_idx[w*16+g] - prev + SCALE_DIFF_ZERO;
                     av_assert1(sfdiff >= 0 && sfdiff <= 2*SCALE_MAX_DIFF);
                 } else if (sce->zeroes[0]) {
                     /** Set global gain to something useful */