[FFmpeg-cvslog] nutdec: fix illegal count check in decode_main_header

[Andreas Cadhalpun]

ffmpeg | branch: release/2.6 | Andreas Cadhalpun <Andreas.Cadhalpun at googlemail.com> | Tue Apr 28 22:37:19 2015 +0200| [2f290cf8815b66849334453273df64ed1d1b1bfe] | committer: Andreas Cadhalpun

nutdec: fix illegal count check in decode_main_header

The existing check has two problems:
 1) i + count can overflow, so that the check '< 256' returns true.
 2) In the (i == 'N') case occurs a j-- so that the loop runs once more.

This can trigger the assertion 'nut->header_len[0] == 0' or cause
segmentation faults or infinite hangs.

Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun at googlemail.com>
Signed-off-by: Michael Niedermayer <michaelni at gmx.at>
(cherry picked from commit 7c24ca1bda2d4df1dc9b2b982941be532d60da21)
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun at googlemail.com>

> http://git.videolan.org/gitweb.cgi/ffmpeg.git/?a=commit;h=2f290cf8815b66849334453273df64ed1d1b1bfe
---

 libavformat/nutdec.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/libavformat/nutdec.c b/libavformat/nutdec.c
index dc17228..6808f35 100644
--- a/libavformat/nutdec.c
+++ b/libavformat/nutdec.c
@@ -294,7 +294,7 @@ static int decode_main_header(NUTContext *nut)
         while (tmp_fields-- > 8)
             ffio_read_varlen(bc);
 
-        if (count == 0 || i + count > 256) {
+        if (count <= 0 || count > 256 - (i <= 'N') - i) {
             av_log(s, AV_LOG_ERROR, "illegal count %d at %d\n", count, i);
             return AVERROR_INVALIDDATA;
         }