[FFmpeg-cvslog] pngdec: check s->last_picture.f->data[0] before using it

[Andreas Cadhalpun]

ffmpeg | branch: release/2.5 | Andreas Cadhalpun <andreas.cadhalpun at googlemail.com> | Sun May  3 16:21:39 2015 +0200| [c3653e2265d1d7f73d3b4596d2f67db12456116a] | committer: Michael Niedermayer

pngdec: check s->last_picture.f->data[0] before using it

This check was removed in commit 08aec6f6, but
s->last_picture.f->data[0] is still used in handle_p_frame_apng
unconditionally.

This fixes a segmentation fault.

Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun at googlemail.com>
Signed-off-by: Michael Niedermayer <michaelni at gmx.at>
(cherry picked from commit 287dbb0771d558b336e377d0594e26c0a6291755)

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

> http://git.videolan.org/gitweb.cgi/ffmpeg.git/?a=commit;h=c3653e2265d1d7f73d3b4596d2f67db12456116a
---

 libavcodec/pngdec.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/libavcodec/pngdec.c b/libavcodec/pngdec.c
index 7e7b285..314cccf 100644
--- a/libavcodec/pngdec.c
+++ b/libavcodec/pngdec.c
@@ -1065,7 +1065,7 @@ exit_loop:
     /* handle p-frames only if a predecessor frame is available */
     ref = s->dispose_op == APNG_DISPOSE_OP_PREVIOUS ?
              s->previous_picture.f : s->last_picture.f;
-    if (ref->data[0]) {
+    if (ref->data[0] && s->last_picture.f->data[0]) {
         if (   !(avpkt->flags & AV_PKT_FLAG_KEY) && avctx->codec_tag != AV_RL32("MPNG")
             && ref->width == p->width
             && ref->height== p->height