avcodec/h264_cabac: Tighten allowed coeff_abs range

Fixes: integer overflows
Reported-by: "Xiaohan Wang (王消寒)" <xhwang@chromium.org>

Based on limits in "8.5 Transform coefficient decoding process and picture
construction process prior to deblocking  filter process"

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit f26a63c4ee)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/h264_cabac.c

@@ -1735,7 +1735,7 @@ decode_cabac_residual_internal(const H264Context *h, H264SliceContext *sl,
 \
             if( coeff_abs >= 15 ) { \
                 int j = 0; \
-                while (get_cabac_bypass(CC) && j < 30) { \
+                while (get_cabac_bypass(CC) && j < 16+7) { \
                     j++; \
                 } \
 \