h264_cabac: Break infinite loops

This fixes out of array reads and/or infinite loops.

30 is the maximum number of bits that can be read into
coeff_abs below.

CC: libav-stable@libav.org
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Martin Storsjö <martin@martin.st>

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++; \
                 } \
 \