[FFmpeg-cvslog] avcodec/mpegvideo_motion: Check P field references

[Michael Niedermayer]

ffmpeg | branch: release/1.2 | Michael Niedermayer <michaelni at gmx.at> | Thu Apr 18 19:53:00 2013 +0200| [a2186a8054428807d7bcd9ee29342e8741177f9c] | committer: Michael Niedermayer

avcodec/mpegvideo_motion: Check P field references

If a reference is unavailable use a field from the current
picture
Fixes null pointer dereference

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni at gmx.at>
(cherry picked from commit 23daee0dcc57b647b9d62d4c905e94acf0c6b8e0)

Signed-off-by: Michael Niedermayer <michaelni at gmx.at>

> http://git.videolan.org/gitweb.cgi/ffmpeg.git/?a=commit;h=a2186a8054428807d7bcd9ee29342e8741177f9c
---

 libavcodec/mpegvideo_motion.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/libavcodec/mpegvideo_motion.c b/libavcodec/mpegvideo_motion.c
index 565f6cb..f443b8e 100644
--- a/libavcodec/mpegvideo_motion.c
+++ b/libavcodec/mpegvideo_motion.c
@@ -812,7 +812,8 @@ static av_always_inline void MPV_motion_internal(MpegEncContext *s,
                                   s->mv[dir][1][0], s->mv[dir][1][1], 8, mb_y);
             }
         } else {
-            if(s->picture_structure != s->field_select[dir][0] + 1 && s->pict_type != AV_PICTURE_TYPE_B && !s->first_field){
+            if(   s->picture_structure != s->field_select[dir][0] + 1 && s->pict_type != AV_PICTURE_TYPE_B && !s->first_field
+               || !ref_picture[0]){
                 ref_picture = s->current_picture_ptr->f.data;
             }