avcodec/cbs_h265_syntax_template: Check num_negative/positive_pics when inter_ref_pic_set_prediction_flag is set

Fixes: out of array access
Fixes: 20446/clusterfuzz-testcase-minimized-ffmpeg_BSF_HEVC_METADATA_fuzzer-5707770718584832

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/cbs_h265_syntax_template.c

@@ -601,6 +601,8 @@ 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],
@@ -630,6 +632,8 @@ 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],