lavfi/subtitles: fix memleak after 6bcb1e1a

Spotted-by: James Almer <jamrial@gmail.com>

libavfilter/vf_subtitles.c

@@ -478,6 +478,7 @@ end:
     av_dict_free(&codec_opts);
     if (dec_ctx)
         avcodec_close(dec_ctx);
+    avcodec_free_context(&dec_ctx);
     if (fmt)
         avformat_close_input(&fmt);
     return ret;