aacenc: psy_3gpp_init(): Fix ath for the first line in each sfb.

Fix the MDCT line to frequency calculation for the first line in each sfb.
Use this value to calculate ATH.

Originally committed as revision 24231 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/aacpsy.c

@@ -138,7 +138,7 @@ static av_cold int psy_3gpp_init(FFPsyContext *ctx) {
         }
         start = 0;
         for (g = 0; g < ctx->num_bands[j]; g++) {
-            minscale = ath(ctx->avctx->sample_rate * start / 1024.0, ATH_ADD);
+            minscale = ath(ctx->avctx->sample_rate * start / 1024.0 / 2.0, ATH_ADD);
             for (i = 1; i < ctx->bands[j][g]; i++)
                 minscale = FFMIN(minscale, ath(ctx->avctx->sample_rate * (start + i) / 1024.0 / 2.0, ATH_ADD));
             coeffs->ath[g] = minscale - minath;