avcodec/xan: Add basic input size check

Fixes: Timeout
Fixes: 71739/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_XAN_WC3_fuzzer-6170301405134848

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

libavcodec/xan.c

@@ -607,6 +607,9 @@ static int xan_decode_frame(AVCodecContext *avctx, AVFrame *frame,
         return AVERROR_INVALIDDATA;
     }
 
+    if (buf_size < 9)
+        return AVERROR_INVALIDDATA;
+
     if ((ret = ff_get_buffer(avctx, frame, AV_GET_BUFFER_FLAG_REF)) < 0)
         return ret;