avcodec/eatqi: Check for minimum frame size

The minimum header is 8 bytes, the smallest bitstream that is passed to
the MB decode code is 4 bytes

Fixes: Timeout (35sec -> 18sec)
Fixes: 15800/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_EATQI_fuzzer-5684154517159936

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/eatqi.c

@@ -131,6 +131,9 @@ static int tqi_decode_frame(AVCodecContext *avctx,
     AVFrame *frame = data;
     int ret, w, h;
 
+    if (buf_size < 12)
+        return AVERROR_INVALIDDATA;
+
     t->avctx = avctx;
 
     w = AV_RL16(&buf[0]);