avcodec/utils: fix leak of subtitle_header on error path

Fixes: memleak
Fixes: 15528/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_STL_fuzzer-5735993371525120
Fixes: 15792/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_SAMI_fuzzer-5737754232619008
Fixes: 16008/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_SSA_fuzzer-5650582821404672

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 923d5c489f)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/utils.c

@@ -1041,6 +1041,7 @@ FF_ENABLE_DEPRECATION_WARNINGS
 
     av_dict_free(&tmp);
     av_freep(&avctx->priv_data);
+    av_freep(&avctx->subtitle_header);
     if (avctx->internal) {
         av_frame_free(&avctx->internal->to_free);
         av_frame_free(&avctx->internal->compat_decode_frame);