avcodec/cbs_h265_syntax_template: Limit num_long_term_pics more strictly

The limit is based on hevcdec.c
Fixes: 20854/clusterfuzz-testcase-minimized-ffmpeg_BSF_HEVC_METADATA_fuzzer-5160442882424832
Fixes: out of array access

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

libavcodec/cbs_h265_syntax_template.c

@@ -1377,7 +1377,7 @@ static int FUNC(slice_segment_header)(CodedBitstreamContext *ctx, RWContext *rw,
                     infer(num_long_term_sps, 0);
                     idx_size = 0;
                 }
-                ue(num_long_term_pics, 0, HEVC_MAX_LONG_TERM_REF_PICS);
+                ue(num_long_term_pics, 0, HEVC_MAX_REFS - current->num_long_term_sps);
 
                 for (i = 0; i < current->num_long_term_sps +
                                 current->num_long_term_pics; i++) {