avcodec/hevc_refs: Check that nb_refs stays <= HEVC_MAX_REFS

Fixes: 58528/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_HEVC_fuzzer-5834725318328320
Fixes: index 16 out of bounds for type 'int [16]'

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

libavcodec/hevc_refs.c

@@ -354,7 +354,7 @@ int ff_hevc_slice_rpl(HEVCContext *s)
                 }
             }
             // Construct RefPicList0, RefPicList1 (8-8, 8-10)
-            if (s->ps.pps->pps_curr_pic_ref_enabled_flag) {
+            if (s->ps.pps->pps_curr_pic_ref_enabled_flag && rpl_tmp.nb_refs < HEVC_MAX_REFS) {
                 rpl_tmp.list[rpl_tmp.nb_refs]           = s->ref->poc;
                 rpl_tmp.ref[rpl_tmp.nb_refs]            = s->ref;
                 rpl_tmp.isLongTerm[rpl_tmp.nb_refs]     = 1;