libavcodec/8bps: Check that line lengths fit within the buffer

Fixes: Timeout
Fixes: undefined pointer arithmetic
Fixes: 50330/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_EIGHTBPS_fuzzer-5436287485607936

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

libavcodec/8bps.c

@@ -68,6 +68,9 @@ static int decode_frame(AVCodecContext *avctx, AVFrame *frame,
     unsigned char *planemap = c->planemap;
     int ret;
 
+    if (buf_size < planes * height *2)
+        return AVERROR_INVALIDDATA;
+
     if ((ret = ff_get_buffer(avctx, frame, 0)) < 0)
         return ret;