aacpsy: correct calculation of minath in psy_3gpp_init

The minimum of the ath(x, ATH_ADD) function depends on ATH_ADD.
This patch uses the first order approximation to determine it.

For ATH_ADD = 4 this results in the value at 3407.06812 (-5.24241638)
not the one at 3410 (-5.24237967).

CC: libav-stabl@libav.org
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
Signed-off-by: Anton Khirnov <anton@khirnov.net>
(cherry picked from commit 110f7f35fb)
Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavcodec/aacpsy.c

@@ -307,7 +307,7 @@ static av_cold int psy_3gpp_init(FFPsyContext *ctx) {
     ctx->bitres.size   = 6144 - pctx->frame_bits;
     ctx->bitres.size  -= ctx->bitres.size % 8;
     pctx->fill_level   = ctx->bitres.size;
-    minath = ath(3410, ATH_ADD);
+    minath = ath(3410 - 0.733 * ATH_ADD, ATH_ADD);
     for (j = 0; j < 2; j++) {
         AacPsyCoeffs *coeffs = pctx->psy_coef[j];
         const uint8_t *band_sizes = ctx->bands[j];