[FFmpeg-cvslog] avcodec/aacdec_fixed: fix invalid shift in predict()

[Michael Niedermayer]

ffmpeg | branch: release/3.1 | Michael Niedermayer <michael at niedermayer.cc> | Fri Aug  4 03:26:30 2017 +0200| [46023f3258f4082cf1aba9b47401bdb137174103] | committer: Michael Niedermayer

avcodec/aacdec_fixed: fix invalid shift in predict()

Fixes: runtime error: shift exponent -2 is negative
Fixes: 2818/clusterfuzz-testcase-minimized-5062943676825600

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 1e443051b277f73b94a2f660d3fd31a1a7beab52)
Signed-off-by: Michael Niedermayer <michael at niedermayer.cc>

> http://git.videolan.org/gitweb.cgi/ffmpeg.git/?a=commit;h=46023f3258f4082cf1aba9b47401bdb137174103
---

 libavcodec/aacdec_fixed.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/libavcodec/aacdec_fixed.c b/libavcodec/aacdec_fixed.c
index ccc82057e1..e7c2d2d299 100644
--- a/libavcodec/aacdec_fixed.c
+++ b/libavcodec/aacdec_fixed.c
@@ -305,8 +305,12 @@ static av_always_inline void predict(PredictorState *ps, int *coef,
     if (output_enable) {
         int shift = 28 - pv.exp;
 
-        if (shift < 31)
-            *coef += (pv.mant + (1 << (shift - 1))) >> shift;
+        if (shift < 31) {
+            if (shift > 0) {
+                *coef += (pv.mant + (1 << (shift - 1))) >> shift;
+            } else
+                *coef += pv.mant << -shift;
+        }
     }
 
     e0 = av_int2sf(*coef, 2);