indeo3: fix data size check

The data offsets are relative to the bistream header, which is 16 bytes
after the start of the data.
Fixes invalid reads with corrupted files.

Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC:libav-stable@libav.org

libavcodec/indeo3.c

@@ -893,8 +893,7 @@ static int decode_frame_headers(Indeo3DecodeContext *ctx, AVCodecContext *avctx,
 
     if (ctx->data_size == 16)
         return 4;
-    if (ctx->data_size > buf_size)
-        ctx->data_size = buf_size;
+    ctx->data_size = FFMIN(ctx->data_size, buf_size - 16);
 
     bytestream2_skip(&gb, 3); // skip reserved byte and checksum