AAC encoder: fix assertion error with prediction

Fixes an assertion error reported in #2686 that happens when
using prediction (either explicitly or implicitly by setting
the AAC main profile), since prediction code would allow
creating new zeroes or removing existing ones, without
properly checking for SF delta violations.

This patch forbids creating/removing zeroes, perhaps an
overly conservative approach, but a safe one. More permissive
and sophisticated approaches may be attempted in the future.

libavcodec/aacenc_pred.c

@@ -257,7 +257,9 @@ void ff_aac_search_for_pred(AACEncContext *s, SingleChannelElement *sce)
     for (sfb = PRED_SFB_START; sfb < pmax; sfb++) {
         int cost1, cost2, cb_p;
         float dist1, dist2, dist_spec_err = 0.0f;
-        const int cb_n = sce->band_type[sfb];
+        const int cb_n = sce->zeroes[sfb] ? 0 : sce->band_type[sfb];
+        const int cb_min = sce->zeroes[sfb] ? 0 : 1;
+        const int cb_max = sce->zeroes[sfb] ? 0 : RESERVED_BT;
         const int start_coef = sce->ics.swb_offset[sfb];
         const int num_coeffs = sce->ics.swb_offset[sfb + 1] - start_coef;
         const FFPsyBand *band = &s->psy.ch[s->cur_channel].psy_bands[sfb];
@@ -279,7 +281,7 @@ void ff_aac_search_for_pred(AACEncContext *s, SingleChannelElement *sce)
             SENT[i] = sce->coeffs[start_coef + i] - sce->prcoeffs[start_coef + i];
         abs_pow34_v(S34, SENT, num_coeffs);
         if (cb_n < RESERVED_BT)
-            cb_p = find_min_book(find_max_val(1, num_coeffs, S34), sce->sf_idx[sfb]);
+            cb_p = av_clip(find_min_book(find_max_val(1, num_coeffs, S34), sce->sf_idx[sfb]), cb_min, cb_max);
         else
             cb_p = cb_n;
         quantize_and_encode_band_cost(s, NULL, SENT, QERR, S34, num_coeffs,
@@ -291,7 +293,7 @@ void ff_aac_search_for_pred(AACEncContext *s, SingleChannelElement *sce)
             sce->prcoeffs[start_coef + i] += QERR[i] != 0.0f ? (sce->prcoeffs[start_coef + i] - QERR[i]) : 0.0f;
         abs_pow34_v(P34, &sce->prcoeffs[start_coef], num_coeffs);
         if (cb_n < RESERVED_BT)
-            cb_p = find_min_book(find_max_val(1, num_coeffs, P34), sce->sf_idx[sfb]);
+            cb_p = av_clip(find_min_book(find_max_val(1, num_coeffs, P34), sce->sf_idx[sfb]), cb_min, cb_max);
         else
             cb_p = cb_n;
         dist2 = quantize_and_encode_band_cost(s, NULL, &sce->prcoeffs[start_coef], NULL,