h264/cabac: check loop index

fix out of array read

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/h264_cabac.c

@@ -1712,7 +1712,7 @@ decode_cabac_residual_internal(H264Context *h, int16_t *block,
 \
             if( coeff_abs >= 15 ) { \
                 int j = 0; \
-                while( get_cabac_bypass( CC ) ) { \
+                while(get_cabac_bypass( CC ) && j<30) { \
                     j++; \
                 } \
 \