fftools/ffmpeg_demux: Don't use fake object with av_opt_eval

The av_opt_eval family of functions emits errors messages on error
and can therefore not be used with fake objects when the AVClass
has a custom item_name callback. The AVClass for AVCodecContext
has such a custom callback (it searches whether an AVCodec is set
to use its name). In practice it means that whatever is directly
after the "cc" pointer to the AVClass for AVCodec in the stack frame
of ist_add() will be treated as a pointer to an AVCodec with
unpredictable consequences.

Fix this by using an actual AVCodecContext instead of a fake object.

Reviewed-by: Anton Khirnov <anton@khirnov.net>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

fftools/ffmpeg_demux.c

@@ -1042,9 +1042,6 @@ static int ist_add(const OptionsContext *o, Demuxer *d, AVStream *st)
     char *codec_tag = NULL;
     char *next;
     char *discard_str = NULL;
-    const AVClass *cc = avcodec_get_class();
-    const AVOption *discard_opt = av_opt_find(&cc, "skip_frame", NULL,
-                                              0, AV_OPT_SEARCH_FAKE_OBJ);
     int ret;
 
     ds  = demux_stream_alloc(d, st);
@@ -1176,18 +1173,20 @@ static int ist_add(const OptionsContext *o, Demuxer *d, AVStream *st)
         (o->data_disable && ist->st->codecpar->codec_type == AVMEDIA_TYPE_DATA))
             ist->user_set_discard = AVDISCARD_ALL;
 
+    ist->dec_ctx = avcodec_alloc_context3(ist->dec);
+    if (!ist->dec_ctx)
+        return AVERROR(ENOMEM);
+
     if (discard_str) {
-        ret = av_opt_eval_int(&cc, discard_opt, discard_str, &ist->user_set_discard);
+        const AVOption *discard_opt = av_opt_find(ist->dec_ctx, "skip_frame",
+                                                  NULL, 0, 0);
+        ret = av_opt_eval_int(ist->dec_ctx, discard_opt, discard_str, &ist->user_set_discard);
         if (ret  < 0) {
             av_log(ist, AV_LOG_ERROR, "Error parsing discard %s.\n", discard_str);
             return ret;
         }
     }
 
-    ist->dec_ctx = avcodec_alloc_context3(ist->dec);
-    if (!ist->dec_ctx)
-        return AVERROR(ENOMEM);
-
     ret = avcodec_parameters_to_context(ist->dec_ctx, par);
     if (ret < 0) {
         av_log(ist, AV_LOG_ERROR, "Error initializing the decoder context.\n");