avcodec/cdgraphics: check buffer size before use

Fixes out of array accesses

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/cdgraphics.c

@@ -289,7 +289,9 @@ static int cdg_decode_frame(AVCodecContext *avctx,
     inst    = bytestream_get_byte(&buf);
     inst    &= CDG_MASK;
     buf += 2;  /// skipping 2 unneeded bytes
-    bytestream_get_buffer(&buf, cdg_data, buf_size - CDG_HEADER_SIZE);
+
+    if (buf_size > CDG_HEADER_SIZE)
+        bytestream_get_buffer(&buf, cdg_data, buf_size - CDG_HEADER_SIZE);
 
     if ((command & CDG_MASK) == CDG_COMMAND) {
         switch (inst) {