avcodec/bintext: Check input size before allocating the input image

Fixes: Timeout
Fixes: 9795/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_XBIN_fuzzer-5768631928487936

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

libavcodec/bintext.c

@@ -146,6 +146,9 @@ static int decode_frame(AVCodecContext *avctx,
     const uint8_t *buf_end = buf+buf_size;
     int ret;
 
+    if ((avctx->width / FONT_WIDTH) * (avctx->height / s->font_height) / 256 > buf_size)
+        return AVERROR_INVALIDDATA;
+
     s->x = s->y = 0;
     if ((ret = ff_reget_buffer(avctx, s->frame)) < 0)
         return ret;