h264dec: use a large enough field for reference list modification values

pic_num can be at most 17-bit, so uint8_t is not sufficient.
Found-By: Bradley Sepos <bradley@bradleysepos.com>
CC: libav-stable@libav.org

libavcodec/h264dec.h

@@ -268,7 +268,7 @@ typedef struct H264SliceContext {
                                          *   according to picture reordering in slice header */
     struct {
         uint8_t op;
-        uint8_t val;
+        uint32_t val;
     } ref_modifications[2][32];
     int nb_ref_modifications[2];