[FFmpeg-cvslog] avcodec/mpeg4videodec: Fix undefined shift in get_amv()

[Michael Niedermayer]

ffmpeg | branch: release/3.3 | Michael Niedermayer <michael at niedermayer.cc> | Sat Sep 15 00:20:38 2018 +0200| [43a80efee600d75b5ae9efe52f36303dabfb81bb] | committer: Michael Niedermayer

avcodec/mpeg4videodec: Fix undefined shift in get_amv()

Fixes: runtime error: shift exponent -1 is negative
Fixes: 9938/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_MPEG4_fuzzer-5653783529914368

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael at niedermayer.cc>
(cherry picked from commit c88afa44c4823aba7b6f4a1b01fd6a4169643c57)
Signed-off-by: Michael Niedermayer <michael at niedermayer.cc>

> http://git.videolan.org/gitweb.cgi/ffmpeg.git/?a=commit;h=43a80efee600d75b5ae9efe52f36303dabfb81bb
---

 libavcodec/mpeg4videodec.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/libavcodec/mpeg4videodec.c b/libavcodec/mpeg4videodec.c
index 319a3809ea..91a0d726f9 100644
--- a/libavcodec/mpeg4videodec.c
+++ b/libavcodec/mpeg4videodec.c
@@ -539,7 +539,7 @@ static inline int get_amv(Mpeg4DecContext *ctx, int n)
         len >>= s->quarter_sample;
 
     if (s->real_sprite_warping_points == 1) {
-        if (ctx->divx_version == 500 && ctx->divx_build == 413)
+        if (ctx->divx_version == 500 && ctx->divx_build == 413 && a >= s->quarter_sample)
             sum = s->sprite_offset[0][n] / (1 << (a - s->quarter_sample));
         else
             sum = RSHIFT(s->sprite_offset[0][n] * (1 << s->quarter_sample), a);