avcodec/fmvc: Check if header fields are available before allocating the image

Fixes: Timeout (15sec -> 0.5sec)
Fixes: 14846/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_FMVC_fuzzer-5068322120400896

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/fmvc.c

@@ -402,6 +402,9 @@ static int decode_frame(AVCodecContext *avctx, void *data,
     AVFrame *frame = data;
     int ret, y, x;
 
+    if (avpkt->size < 8)
+        return AVERROR_INVALIDDATA;
+
     if ((ret = ff_get_buffer(avctx, frame, 0)) < 0)
         return ret;