avformat/oggdec: Disable mid stream codec changes

Fixes: 22082/clusterfuzz-testcase-minimized-ffmpeg_DEMUXER_fuzzer-5688619118624768
Fixes: crash from V-codecs/Theora/theora_testsuite_broken/multi2.ogg

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

libavformat/oggdec.c

@@ -226,9 +226,10 @@ static int ogg_replace_stream(AVFormatContext *s, uint32_t serial, char *magic,
         return AVERROR_INVALIDDATA;
     }
 
-    /* We only have a single stream anyway, so if there's a new stream with
-     * a different codec just replace it */
     os = &ogg->streams[0];
+    if (os->codec != codec)
+        return AVERROR(EINVAL);
+
     os->serial  = serial;
     os->codec   = codec;
     os->serial  = serial;