avcodec/hevcdec: Fix memleak of a53_caption

Fixes: 15295/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_HEVC_fuzzer-5675655187922944

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit ef50cf7b32)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/hevcdec.c

@@ -3336,6 +3336,8 @@ static av_cold int hevc_decode_free(AVCodecContext *avctx)
 
     ff_h2645_packet_uninit(&s->pkt);
 
+    ff_hevc_reset_sei(&s->sei);
+
     return 0;
 }
 
@@ -3529,6 +3531,7 @@ static void hevc_decode_flush(AVCodecContext *avctx)
 {
     HEVCContext *s = avctx->priv_data;
     ff_hevc_flush_dpb(s);
+    ff_hevc_reset_sei(&s->sei);
     s->max_ra = INT_MAX;
     s->eos = 1;
 }