cbs_h265: Ensure that a predicted RPS doesn't contain too many pictures

If the RPS we are predicting from has maximum size then at least one of
the pictures in it must be discarded before adding the current one.

Also revert 588114cea4, which added
now-redundant checks for the special case of a too-large RPS with all
pictures being in the same direction from the current one.

libavcodec/cbs_h265_syntax_template.c

@@ -522,7 +522,7 @@ static int FUNC(st_ref_pic_set)(CodedBitstreamContext *ctx, RWContext *rw,
         infer(inter_ref_pic_set_prediction_flag, 0);
 
     if (current->inter_ref_pic_set_prediction_flag) {
-        unsigned int ref_rps_idx, num_delta_pocs;
+        unsigned int ref_rps_idx, num_delta_pocs, num_ref_pics;
         const H265RawSTRefPicSet *ref;
         int delta_rps, d_poc;
         int ref_delta_poc_s0[HEVC_MAX_REFS], ref_delta_poc_s1[HEVC_MAX_REFS];
@@ -538,18 +538,28 @@ static int FUNC(st_ref_pic_set)(CodedBitstreamContext *ctx, RWContext *rw,
         ref_rps_idx = st_rps_idx - (current->delta_idx_minus1 + 1);
         ref = &sps->st_ref_pic_set[ref_rps_idx];
         num_delta_pocs = ref->num_negative_pics + ref->num_positive_pics;
+        av_assert0(num_delta_pocs < HEVC_MAX_DPB_SIZE);
 
         flag(delta_rps_sign);
         ue(abs_delta_rps_minus1, 0, INT16_MAX);
         delta_rps = (1 - 2 * current->delta_rps_sign) *
             (current->abs_delta_rps_minus1 + 1);
 
+        num_ref_pics = 0;
         for (j = 0; j <= num_delta_pocs; j++) {
             flags(used_by_curr_pic_flag[j], 1, j);
             if (!current->used_by_curr_pic_flag[j])
                 flags(use_delta_flag[j], 1, j);
             else
                 infer(use_delta_flag[j], 1);
+            if (current->use_delta_flag[i])
+                ++num_ref_pics;
+        }
+        if (num_ref_pics >= HEVC_MAX_DPB_SIZE) {
+            av_log(ctx->log_ctx, AV_LOG_ERROR, "Invalid stream: "
+                   "short-term ref pic set %d "
+                   "contains too many pictures.\n", st_rps_idx);
+            return AVERROR_INVALIDDATA;
         }
 
         // Since the stored form of an RPS here is actually the delta-step
@@ -601,8 +611,6 @@ static int FUNC(st_ref_pic_set)(CodedBitstreamContext *ctx, RWContext *rw,
             }
         }
 
-        if (i > 15)
-            return AVERROR_INVALIDDATA;
         infer(num_negative_pics, i);
         for (i = 0; i < current->num_negative_pics; i++) {
             infer(delta_poc_s0_minus1[i],
@@ -632,8 +640,6 @@ static int FUNC(st_ref_pic_set)(CodedBitstreamContext *ctx, RWContext *rw,
             }
         }
 
-        if (i + current->num_negative_pics > 15)
-            return AVERROR_INVALIDDATA;
         infer(num_positive_pics, i);
         for (i = 0; i < current->num_positive_pics; i++) {
             infer(delta_poc_s1_minus1[i],