avcodec/vp3: Check input amount in theora_decode_header()

Fixes: Timeout
Fixes: 29226/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_THEORA_fuzzer-6195092572471296

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Reviewed-by: Peter Ross <pross@xvid.org>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/vp3.c

@@ -2875,6 +2875,9 @@ static int theora_decode_header(AVCodecContext *avctx, GetBitContext *gb)
     int ret;
     AVRational fps, aspect;
 
+    if (get_bits_left(gb) < 206)
+        return AVERROR_INVALIDDATA;
+
     s->theora_header = 0;
     s->theora = get_bits(gb, 24);
     av_log(avctx, AV_LOG_DEBUG, "Theora bitstream version %X\n", s->theora);